July 27, 2012 Mammoth - Mammoth Community Water District...Mammoth Lakes and approximately 2 miles southwest of the Mammoth/Yosemite Airport. Laurel Pond is located along the southern

July 27, 2012

Mammoth Community Water District Greg Norby, General Manager PO Box 597 Mammoth Lakes, CA 93546

Subject: Laurel Pond Water Budget and Evaluation of Recycling Alternatives

Dear Mr. Norby:

The Mammoth Community Water District (MCWD) provides municipal water, wastewater, and recycled water service to the Town of Mammoth Lakes. The MCWD’s wastewater treatment plant discharges secondary‐treated effluent to Laurel Pond, a small and natural water body (MCWD, 2011), located approximately 3.5 miles southeast of the treatment plant on U.S. Forest Service (USFS) land, as shown in Figure 1. Laurel Pond is permitted for effluent disposal by the Lahontan Regional Water Quality Control Board (RWQCB).1 Beginning in 1985, treated effluent was conveyed from the treatment plant to Laurel Pond by a gravity pipeline. Laurel Pond is a terminal water body: there is no discharge from Laurel Pond to Mammoth Creek. Prior to June 1985, Laurel Pond was an ephemeral pond, with pond size dependent on natural inflow components, including precipitation in the Laurel Pond watershed, spring discharge, and groundwater inflow; and discharge components, such as evaporation and infiltration (CH2M HILL, 2001; WEI, 2009a; MCWD, 2011). Since 1985, the natural inflow components have been augmented with treatment plant effluent, causing the pond to become perennial and gradually increase in size. In 2009, the MCWD completed construction of its recycled water treatment and conveyance facilities and began providing irrigation supply to the Sierra Star Golf Course. The MCWD will begin serving a second golf course, Snowcreek, in 2015. The District is initiating an updated recycled water master plan to evaluate future increases in recycled water use and related demands. When making recycled water deliveries for irrigation uses, the MCWD reduces its discharge to Laurel Pond. The reduction in discharge occurs primarily during the irrigation season (May through September). Current and future recycled water use and the resulting effluent discharge reduction depends on the number of recycled water customers and their demands. The MCWD has an agreement2 with the USFS to utitlize the effluent discharge in a 1California Regional Water Quality Control Board Lahontan Region Board Order No. 6‐91‐22 WDID No. 6B260103001 2Memonrandum of Agreement Between the U.S. Forest Service, Inyo National Forest, and Mammoth Community Water District (1983).

Laurel Pond Water Budget and Evaluation of Recycling Alternatives July 27, 2012

Greg Norby Page 2 of 13

beneficial manner to maintain a waterfowl habitat in the vicinity of Laurel Pond, and the MCWD determined that a pond area of 18 acres is the minimum area needed to support the waterfowl habitat (MCWD and Bauer, 1998). The objectives of this investigation were to develop and calibrate a hydrologic model of Laurel Pond and to apply the calibrated model to estimate the time history of the pond size for various future recycled water use alternatives. CH2M Hill developed a similar model that used the seven‐year hydrologic period of 1994 through 2001 to evaluate the pond hyrology. In the investigation reported herein, Wildermuth Environmental (WEI) developed a daily simulation model for Laurel Pond for the period of 1985 through 2011, a period of 26 years. The longer calibration period resulted in a more robust and detailed model and confirmed important infiltration properties that change with time and pond size for use in the planning simulations. The model generated estimates of pond volume, surface area, and key hydrologic parameters over the simulation period for recycled water resue planning alternatives.

Site Description As Figure 1 shows, Laurel Pond is located approximately 5.5 miles southeast of the Town of Mammoth Lakes and approximately 2 miles southwest of the Mammoth/Yosemite Airport. Laurel Pond is located along the southern margin of the Long Valley Caldera floor within the Mammoth Basin. Figure 2 is a geologic map of the area and shows that the geologic materials surrounding the pond consist of Quaternary age unconsolidated younger and older alluvium and Quaternary age talus deposits that flank Laurel Pond directly to the south (Bailey, 1989). Figure 3 is a hydrogeologic cross‐section developed from four monitoring wells that surround Laurel Pond. The lithologic logs show that the sub‐surface geology consists of alluvial materials, up to 15 feet (ft) thick, interbedded with thin silt and clay lenses, ranging from 2 to 8 ft thick, and volcanic materials ranging from 5 to 10 ft thick. Both the clay and and clayey silt units do not appear to be laterally extensive beneath Laurel Pond. Likewise, the volcanic materials encountered in the LP No. 1 and 4 boreholes are likely the subsurface lobe of the Quaternary porphyritic basalt shown in Figure 2. Volcanic materials were not identified in LP No. 1 or 3. Appendix A, included on the appendix CD, contains the borehole logs for the Laurel Pond monitoring wells.

Laurel Pond Water‐Budget The hydrology of Laurel Pond was simulated using the Router module incorporated into the Wasteload Allocation Model (WLAM), which was developed by WEI and extensively used to simulate the Santa Ana River system in Southern California (WEI, 2009b and 2010). In the WLAM, Laurel Pond is represented as a reservoir, and reservoirs are represented by rating curves that relate water surface elevation to surface area and storage (depth‐area‐capacity curves), discharge through outlet works and spillways, evaporation, and infiltration rates. The daily mass balance equation for a reservoir is expressed as:



St ‐ S t‐1 = Irt + Igwt +Ipt + Int ‐ Oit ‐ Oowt ‐ Et

Where:

St is the storage at the end of time t S t‐1 is the storage at the end of time t‐1 Irt is the runoff inflow during time step t‐1 to t Igwt is the groundwater inflow during time step t‐1 to t Ip is the precipitation on the pond during time step t‐1 to t Int is the non‐tributary inflow during time step t‐1 to t Et is the pond evaporation during time step t‐1 to t Qit is the infiltration during time step t‐1 to t Qowt is the discharge through outlet works during time step t‐1 to t

Since Laurel Pond is a terminal water body, the outlet works and spillway rating curves are not used. The computational procedure used in the Router Module is the modified Puls method. The development of the depth‐area‐capacity curves and each of the inflow and outflow components present at Laurel Pond are discussed below. Figure 4 shows the inflow and outflow components.

Laurel Pond Area Time History Quantifying the extent and area of Laurel Pond during the historical period of 1985 through 2011 was completed by digitizing the observed shorelines from aerial photographs. Nineteen aerial photos were obtained for this period and examined to determine the shoreline extent and area of Laurel Pond. Table 1 includes the aerial image date, area, and photo source used in the aerial photo investigation. Figure 5 shows the extent of Laurel Pond in 1986, 1988, 1993, 1998, 2005, and 2010. The pond area was also measured by MCWD staff in 2001, 2004, and 2006 by mapping the pond boundary extent with a differential GPS. On September 14, 2011, WEI staff mapped the pond boundary extent with a handheld GPS. The mapped pond areas between 1986 and 2011 were used as calibration targets for the Laurel Pond hydrology model calibration. Appendix B, included on the appendix CD, contains all of the aerial photos analyzed to determine shoreline extent. Effluent discharge to Laurel Pond remained fairly constant from 1985 through 2011 with annual average daily discharge ranging from about 1.2 to 1.4 mgd. Over the last 26 years, with effluent flow being fairly constant, the Laurel Pond area has increased in size from a minimum of approximately 30 acres (September 1985) to a maximum of approximately 110 acres (September 2006). The pond area ranged from about 30 to 60 acres during the 1985 through 1993 period and, thereafter, grew in size to range from about 70 to 110 acres. This long term increase in average surface area under relatively steady effluent discharge indicates that infiltration has moved to the outer perimeter of the pond due to clogging of the pond bottom in the pond’s central area.



Depth‐Area‐Capacity Curves for Laurel Pond On September 13 and 14, 2011, WEI conducted a detailed bathymetric survey to develop a depth‐area‐capacity curve for Laurel Pond. During this survey, the area of Laurel Pond was about 94 acres. Bathymetric data were obtained using a combination of GPS and GIS methods: the bathymetric survey was conducted by boat, using a Hummingbird® 385ci Combo GPS‐Fishfinder. The table below summarizes the data that were collected and post‐processed. The post‐processed data points in conjunction with manual depth readings taken along the pond’s shoreline were used to create the bathymetric map and contours shown in Figure 6. The complete set of bathymetry data is included in Appendix C (see appendix CD).

Manual Depth Measurement Points

Sonar Depth Measurement Points

Points Used to Define Bathymetry

145 480,631 31,633

Manual depth measurements, sonar measurements, shoreline delineation, and waterfowl mound delineations were combined to create a triangular irregular network (TIN) surface using ArcMap’s Spatial Analyst Tool. Moreover, ArcMap’s 3D Analyst Surface Volume Calculator Tool was used to calculate the area and volume of Laurel Pond below the water surface elevation. Pond elevation was determined using GPS‐tracked shoreline locations and a 5‐meter digital terrain map (DTM) survey conducted by Intermap Technologies, Inc. (2004). Area and volume data were used to create the depth‐area‐capacity curves for Laurel Pond, shown in Figure 7. The depth‐area‐capacity curves were extended to a pond depth of 20 ft by merging the 2004 DTM survey and the 2011 bathymetric survey data. Extending the depth‐area‐curves beyond the 2011 bathymetric survey limit was necessary to in order to simulate storage potential levels in excess of the observed 2011 storage for model calibration and for evaluating future recycled water reuse alternatives.

Inflow Components Wastewater Discharge to Laurel Pond. Daily discharge estimates of the MCWD’s recycled water discharge to Laurel Pond began in June 1985. Discharge records were obtained from the Surface Water Monitoring Reports on file at the RWQCB for the 1985 through 1995 period and from the MCWD for the 1995 through 2011 period. Discharge data are missing for eleven months over the 26‐year period. The missing discharge data were estimated by interpolation from historical data. Table 2 summarizes historical discharge to Laurel Pond. Effluent discharge to Laurel Pond remained fairly constant from 1985 to 2011 with annual average daily discharge ranging from about 1.2 to 1.4 mgd. The average annual effluent discharge is about 1,450 acre‐ft. The annual time history of effluent discharged to Laurel Pond is shown graphically in Figure 8. Appendix C includes estimates of the MCWD’s daily treatment plant discharge data to Laurel Pond from June 1985 to September 2011.



Precipitation over Laurel Pond. A complete daily precipitation record for Laurel Pond was created by combining high‐quality spatial climate data from the 800‐meter grid PRISM3 (Parameter‐elevation Regressions on Independent Slopes Model) dataset and daily precipitation data from the Bridgeport Station (National Climatic Data Center, Station No. 1072). Figure 9 shows the location of the Bridgeport Station and the PRISM grid cell. A relationship was developed between the monthly PRISM data and the Bridgeport precipitation data to convert the monthly PRISM data into daily data over the Laurel Pond area. The equation used to estimate daily precipitation over Laurel Pond is:

PLPt = PBPt * [PPLP/PPB] Where:

PLPt is the precipitation over Laurel Pond during time step t‐1 to t PBPt is the precipitation at Bridgeport during time step t‐1 to t PPLP is the monthly precipitation for 800‐meter PRISM grid over Laurel Pond PPB is the monthly precipitation over Bridgeport gage

The conversion of monthly PRISM data into daily values provides a temporally and spatially complete record for precipitation over Laurel Pond. Appendix C contains the PRISM/Bridgeport hybrid precipitation data used in the hydrologic model. Figure 9 also shows other precipitation stations near Laurel Pond. These stations were considered and reviewed but not utilized in the Laurel Pond hydrologic model. The inset table in Figure 9 summarizes each station’s location, data frequency, and reason for exclusion from the hydrologic model. Laurel Spring Discharge to Laurel Pond. Laurel Springs is a complex of at least two springs: Laurel Spring and Laurel Spring West (J. Pedersen, personal communication, October 4, 2011). The spring complex is located approximately 4,000 ft west of Laurel Pond, as shown in Figure 1. Discharge was measured intermittently by the USFS and the US Geological Survey (USGS) between 1983 and 1995. Due to the complexity of the spring system and vandalism, the discharge data are incomplete and unreliable (J. Pedersen, personal communication, October 4, 2011). For this reason, spring flow is included as a component of the aggregate natural inflow (spring, storm water/snowmelt, and groundwater inflow), described below. Groundwater Inflow to Laurel Pond. Groundwater level data were obtained from the MCWD and the RWQCB for wells LP Nos. 1 through 4 for the 1986 to 2011 period. Measured groundwater elevations west of Laurel Pond suggest that groundwater inflow to Laurel Pond could occur when groundwater levels exceed Laurel Pond’s water surface elevation. Annual groundwater inflow to Laurel Pond is difficult to quantify due to variability in the pond size and water surface elevation, uncertainty in the underlying geology, an incomplete characterization

3 PRISM Climate Group, Oregon State University, http://prism.oregonstate.edu.



of groundwater levels adjacent to the pond, and uncertainty in the hydraulic parameters of the underlying aquifer. For this reason, groundwater inflow to Laurel Pond is included as a component of the aggregate natural inflow (spring, storm water/snowmelt, and groundwater inflow), described below. Storm Water and Snowmelt Inflow. Surface water enters Laurel Pond as storm water and snowmelt inflow. As of this report, surface water inflow to Laurel Pond is not measured; therefore, it had to be estimated indirectly in calibration. For this reason, storm water and snowmelt inflow to Laurel Pond is included as a component of the aggregate natural inflow (spring, storm water/snowmelt, and groundwater inflow), described below. Aggregate Natural Inflow to Laurel Pond. The aggregate natural inflow to Laurel Pond is the sum of the storm water inflow plus snowmelt inflow, spring flow, and groundwater inflow, and was estimated from the following equation:

Inat = K * QMC395t * [PLPW / PMCW395] Where:

Inat is the aggregate natural inflow during time step t‐1 to t K is the scaling factor initially bounded from 0 to 2 QMC395t is the Mammoth Creek discharge during time step t‐1 to t PMCW395 is the cumulative seasonal precipitation on Mammoth Creek watershed

tributary to US395 through March4 based from PRISM data PLPW is the cumulative seasonal precipitation on Laurel Pond watershed through

March based from PRISM data Scaling factor K was determined in calibration. This is an important component of inflow to Laurel Pond, and the hydrologic model is demonstratively sensitive to its estimation. This term will be discussed in more detail below.

Outflow Components Evaporation from Laurel Pond. Local evaporation data is limited to a land evaporation pan that is located at Crowley Lake and maintained by the Los Angeles Department of Water and Power (LADWP). The location of this station is shown in Figure 9. Monitoring began in 1988, and the frequency of measurement varies from daily to weekly and is seasonal with measurements obtained only during non‐freezing months (approximately May to November). In order to compile a complete and daily evaporation record for Laurel Pond during the calibration period, the seasonal evaporation data recorded at Crowley Lake were compared to evapotranspiration data from CIMIS Station Bishop Number 35.5 The correlation between evaporation and

4 The resulting natural inflow to Laurel Pond starts on April 1st and carries through the following March 30th. 5 http://wwwcimis.water.ca.gov/cimis/frontStationDetailData.do?stationId=35.



evapotranspiration for the two sites is good with a coefficient of determination of 0.72.6 A linear relationship between evaporation and evapotranspiration was developed to estimate missing Crowley Lake evaporation data from the evapotranspiration data recorded at CIMIS Station Bishop Number 35. Table 3 compares the average evaporation rate (in inches/year) used for Laurel Pond to various other evaporation estimates in Mono and Inyo Counties. Moreover, the estimated evaporation rate of 43 inches/yr for Laurel Pond compares closely to other sources, such as June Lake (38 inches/yr), Grant Lake (43 inches/yr), and Long Valley Dam at Crowley Lake (41 inches/yr). Appendix C contains the Crowley Lake/Bishop Number 35 hybrid precipitation data used in the Laurel Pond hyrologogic model. Pond Infiltration. Infiltration in Laurel Pond varies during the historical period due to clogging of the pond bottom, caused by fine‐grained materials deposited by natural inflow, treatment plant effluent, and from bacterial and macrophyte plant growth and breakdown. Both fine‐grained material deposition and bacterial/macrophyte plant growth and breakdown were significant features of the natural pond hydrology and likely influenced pond infiltration prior to the effluent discharge beginning in 1985 (Gram/Phillips, 1982). Due to the expansion of the pond area, which results from natural seasonal inflow and its subsequent decline at the end of the natural inflow period, the infiltration rate varies as well. On September 13 and 14, 2011, field oberservations at confirmed the presence of a clayey, organic‐rich, mushy pond bottom, as well as macrophyte plant growth throughout most of the pond area. However, the observed area surrounding the edge of the pond consisted of coarse‐grained, permeable soil. As the pond area expands and contracts over these two general zones of relatively high and low soil permeability, both the average rate and overall quantity of annual infiltration are estimated to vary significantly. Gram/Phillips (1982) estimated infiltration rates to range from 0.14 to 0.3 ft/day (0‐5 ft pond depth), based on infiltrometer tests that, in practice, do not scale up to prototype infiltration rates. Infiltration rates decline with continued inundation due to clogging. There is a clear trend towards increasing pond size over time due to the clogging of pond bottom soils caused by the factors discussed above. Typically, the following occurs in ponds with continuous inundation:

The infiltration rates in the central part of the pond trend towards zero.

The pond grows larger over time, and infiltration occurs on pond margins. That is, the infiltration rate is variable over time and area. This spatial‐temporal infiltration relationship was established during model calibration.

Laurel Pond Model Calibration Laurel Pond’s response to historical natural inflow and treatment plant discharge was simulated using the WLAM Router Module. The initial calibration period began on June 1, 1985 (initial

6 The coefficient of determination is the fraction of the variance in the dependent variable that is explained by the variance in the independent variable and numerically equal to the square of the linear correlation coefficient.



pond storage was assumed to be zero) and continued through June 30, 2011. Review of the measured pond area time‐series data indicated that after 1997, the pond likely entered into a state of dynamic equilibrium. The calibration period was subsequently adjusted to 1998 through 2011. The calibrated infiltration rates associated with the 1998 through 2011 period were used in planning simulations. A sensitivity analysis was conducted to determine which model parameters were the most sensitive and to focus the calibration effort. Sensitivity for model parameter j was estimated as:

Si = [Akt – Aet (x1, x2,..., xi+xi,…, xn)]2 / [xi] Where:

Si is the sensitivity estimate for parameter xi Akt is the measured pond area at time t Aet (x1, …, xn) is the model estimated pond area at time t based on parameter set x1, … xn xi is the model parameter xi

xi is the perturbation of parameter xi Tested model parameters include infiltration rate, evaporation, and the discharge scaling parameter, K, which is used in the estimate of aggregate natural inflow. The sensitivity of each parameter was tested by perturbing each parameter by ten percent. The results of the sensitivity analysis indicated that the evaporation rate was not sensitive and that scaling parameter K used in estimating aggregate natural inflow and the infiltration rate were very sensitive. Consequently, scaling parameter K and the infiltration rate were the only two model parameters addressed in calibration. A trial and error calibration was done by incrementally adjusting scaling parameter K and the depth‐specific infiltration rates until no significant reduction in the sum of the squares error could be achieved. The sum of the squares error is a measure of how well the calibrated model matches observed pond area and is estimated from:

= [Akt – Aet (x1, …, xn)]2

At the conclusion of the calibration process, the coefficient of determination (R2) was 0.64, indicating that the calibrated model can explain about 64 percent of the variability exhibited in the observed pond area measurements. This is the best result that can be achieved given the data available for calibration. The scaling factor K was calibrated to 0.8. The calibrated infiltration rates are 0.04 ft/day for pond elevations below 7,125 ft‐msl, 0.1 ft/day for pond elevations between 7125 and 7,127 ft‐msl, and 1 ft/day for areas of the pond with elevations greater than 7,127 ft‐msl.



Figure 10 shows the simulated Laurel Pond area compared to the observed pond area for the calibration period. Discrepancies between the simulated and observed pond areas are attributed to differences between the estimated aggregate natural inflow and actual natural inflow. The assumption of uniform infiltration rates for selected parts of the pond could also cause discrepancies. These are data challenges that can be overcome by implementing a pond stage and improved groundwater level monitoring program. Table 4 contains an annual water balance for the calibration period of 1998 through 2010. Treatment plant effluent discharge to Laurel Pond is about 71 percent of the average total inflow of about 2,060 acre‐ft per year (acre‐ft/yr) with precipitation over the pond and natural inflow contributing about 6.5 and 22.5 percent, respectively. Pond outflow averages about 2,050 acre‐ft/yr with infiltration and evaporation accounting for about 84 and 16 percent of pond outflow, respectively. The end of year storage averaged about 174 acre‐ft.

Response of Laurel Pond to MCWD Recycling Alternatives The calibrated model was used to simulate the response of Laurel Pond to the selected MCWD recycled water reuse alternatives. WEI and MCWD staffs developed three recycling alternatives consistent with the MCWD 2010 Urban Water Management Plan (MCWD, 2011). These alternatives include low, medium, and high recycled water reuse rates. Each alternative was evaluatated for 2015 and 2030. Table 5 lists the projected recycled water production for each planning year, the projected recycled water reuse under the the low, medium, and high reuse rates, and the amount of effluent water discharged to Laurel Pond, computed as the difference between total recycled water production and projected recycled water reuse. Recycled water was assumed to be diverted at the MCWD treatment plant to the non‐potable system at a uniform rate during the period of May through September. This diversion resulted in a decrease in the MCWD recycled water being discharged to Laurel Pond during the same period. Each planning alternative listed in Table 5 was simulated with the calibrated Laurel Pond model to estimate the time history of the pond area for the hydrologic period of 1986 through 2011. Figures 11a and 11b show the model‐projected Laurel Pond surface area time histories for the recycled water reuse alternatives for planning years 2015 and 2030, respectively. Recall that the minimum pond area the MCWD committed to maintain under the USFS land use agreement is 18 acres. For all alternatives, the projected pond area is greater than the 18‐acre minimum. Table 6 shows the pond areas for the 2015 and 2030 planning years and reuse alternatives. Under the 2015 conditions, the Laurel Pond surface area is projected to reach a minimum of approximatley 50 acres, a maximum of 114 acres, and an average of about 87 acres. At buildout conditions in 2030, and under the most aggressive reuse alternative, the Laurel Pond surface area is projected to reach a minimum of about 47 acres. Under the least aggressive alternative in 2030, the minimum surface area is projected to be about 70 acres. The maximum projected surface area for all three reuse alternatives in 2030 is 120 acres, with the average ranging narrowly between 94 and 99 acres. The variability in pond surface area is caused by the



temporal and seasonal variability of natural inflow and the seasonal diversion of recycled water away from Laurel Pond to the MCWD non‐potable system.

Conclusions and Recommendations

Calibration Conclusions

1. All available and useful hydrologic data were compiled and used to develop the 2012 Laurel Pond Model. The model used to simulate Laurel Pond is the Router Module from the Wasteload Allocation Model.

2. The initial calibration period was set to 1985 through 2010. Review of the time history of recycled water inflow and pond areas showed that the estimated infiltration rates were not constant over the pond, the infiltration rates in the deepest (central) parts of the pond became negligible, and the overall infiltration rates on the pond perimeter were stable. The calibration period was revised to 1998 through 2010.

3. A sensitivity analysis was conducted and the most sensitive variables were aggregated natural inflow and the pond infiltration rate. Pond evaporation was not a sensitive variable in the model.

4. The largest inflow component in the calibration period was recycled water inflow at 71 percent. The largest outflow component was infiltration at 84 percent.

5. The model was calibrated for the 1998 through 2010 period and achieved a R2 of 0.64. An R2 of 0.7 is considered good.

6. The calibration could be improved if the estimated aggregate natural inflow hydrograph used in calibration could be replaced with measured inflow. This could be done by installing a staff gage with instrumentation in the pond. The aggregate natural inflow hydrograph would be derived from the continuity equation. The pond elevations from the staff gage would replace the observed areas as the calibration target. Finally, if the groundwater level measurements in monitoring well LP No. 4 were to correletate strongly with the new pond stage data, the aggregate natural inflow hydrograph could be estimated for the entire joint record of the new pond stage data and the measured groundwater levels at LP No. 4.

7. Errors in pond surface area (and therefore pond storage) could have been introduced from the estimation of pond surface area from aerial photos. The types of errors could include: inaccurate aerial photo dates, poor photo quality, georeferencing, or difficulty in distringuishing the actual shoreline (i.e., marshy surface versus water).

Future projection conclusions

1. The calibrated model was then used to estimate the future pond elevation and surface area time histories for various recycled water reuse alternatives in 2015 and 2030, using the hydrologic period of 1985 through 2010.

2. The results of the planning simulations were converted to time history charts that show the predicted pond surface area for the hydrologic period of 1985 through 2010 and



repeating annual cycles of discharge and reuse. For example, to characterize the impact of a planned level of recycled water reuse for 2015, the entire period of 1985 through 2010 was simulated using the 2015 level of reuse (see Figure 11a). The 18‐acre minimum pond area is also displayed on the same chart for comparison to the projected pond area.

3. In all cases, the projected pond area time series for all levels of reuse is significantly greater than the 18‐acre minimum pond area constraint.

4. The modeling results suggest, subject to the limitations stated below, that if the MCWD reuses its recycled water in the ranges simulated in this study, the Laurel Pond surface area will remain greater than 18 acres.

Model Limitations

1. The detail in the model is limited by infrequent determinations of pond elevation (currently indirectly determined by measuring area from aerial photos or walking the shoreline with a differential GPS) and no measurement of the aggregate natural inflow. It is unclear how these data deficiencies affect the model predictions. Both of these deficiencies could be overcome by installing a stage sensor in the pond and continuously recording the stage.

2. There are no direct measurements of evaporation and precipitation in the Laurel Pond watershed, and therefore estimates for these variables were developed from remote stations using regional analysis.

3. The existing MCWD’s monitoring wells at the Laurel Pond are not adequate to characterize groundwater inflow to Laurel Pond, to validate the pond infiltration rate, or to characterize groundwater levels and quality in the vicinity of the pond.

Recommendations

1. The MCWD should install a pond stage sensor and equip it to record the stage every 15 minutes. The sensor would need to be fixed and surveyed so that observed stage data can be converted to elevation data. The MCWD should download these data monthly.

2. The MCWD should consider installing a weather station that can record precipitation over Laurel Pond. These data could be used to locally adjust precipitation estimates from other precipitation stations. The weather station would record other parameters to enable the estimate of daily pond evaporation and potential evapotranspiration for the high groundwater areas adjacent to the pond.

3. The model should be recalibrated in three years if newly acquired pond stage data correlates strongly with the measured groundwater levels in LP No. 4. Otherwise, the model should be recalibrated after the MCWD acquires at least five years of new stage data.

4. Additional monitoring wells need to be constructed deeper and outside of the pond perimeter to the north and east of the pond to characterize pond infiltration and its effect on groundwater quality. West of the pond, additional monitoring wells need to be constructed to characterize groundwater inflow to the pond and particularly its



frequency of occurrence. In aggregate, these new monitoring wells will define the groundwater flow field. These wells should be equipped with pressure tranducers with integrated data loggers, and groundwater levels should be observed every 15 minutes. The reference measuring points should be surveyed such that the measured depth to water can be converted to elevation. The MCWD should download the data monthly.

Section 6 References Bailey, R. A. (1989). Geologic Map of Long Valley Caldera, Mono‐Inyo Craters Volcanic Chain,

and Vicinity, Mono County, California, U.S. Geological Survey Miscellaneous Investigations Map I‐1933, scale 1:62,500. USGS.

CH2M HILL. (2001). Laurel Pond Hydrologic Evaluation. Sacramento: CH2MHILL. Gram/Phillips and Associates, Inc. (1982). Status Report – Laurel Pond Effluent Disposal Area.

Pasadena: Gram/Phillips and Associates, Inc. Mammoth Community Water District, Bauer Environmental Services. (1998). Draft

Environmental Impact Report and Environmental Assesment: Mammoth Community Water District Proposed Relcaimed Water Project SCH #95121029. Mammoth Lakes. MCWD

Mammoth Community Water District. (2011). Urban Water Management Plan. Mammoth

Lakes: MCWD State Water Resources Control Board. (2002). Watershed Management Initiative (WMI) Chapter

RB 6. Sacramento: SWRCB Wildermuth Environmental, Inc. (2009a). Mammoth Basin Groundwater Model Report. Lake

Forest: Wildermuth Environmental, Inc. Wildermuth Environmental, Inc. (2009b). Santa Ana Watershed Project Authority 2008 Santa

Ana River Wasteload Allocation Model Report. Lake Forest: Wildermuth Environmental, Inc.

Wildermuth Environmental, Inc. (2010). Chino Basin Watermaster 2010 Recharge Master Plan Update (Appendix C). Lake Forest: Wildermuth Environmental, Inc.



We appreciate the opportunity to serve the Mammoth Community Water District on this important and timely project. Please call me or Mike Blazevic if you have any questions or concerns.

Wildermuth Environmental, Inc.

Mark J. Wildermuth, PE President and Principal Engineer

Mike Blazevic, PGStaff Scientist II

Surface Area

(acres)

9/2/1985 31 USGS (NHAP)

8/30/1986 56 USGS (NHAP)

7/5/1987 38 USGS (NAPP)

8/7/1988 38 USGS (NAPP)

6/23/1989 40 MCWD

8/23/1993 52 MCWD

9/25/1993 51 USGS (NHAP)

10/9/1995 70 USGS

8/26/1998 109 USGS

8/09/2001† 87 MCWD

9/27/2002 75 Google

7/16/2003 73 MCWD

7/27/2004† 71 MCWD

8/11/2004† 73 MCWD

8/26/2005 85 MCWD

9/08/2006† 111 MCWD

8/13/2009 86 NAIP

7/17/2010 88 NAIP

9/14/2011† 94 WEI

Table 1Laurel Pond Historical Surface Areas

Image Date Source

†MCWD or WEI used a differential GPS or handheld GPS to walk the Laurel Pond shoreline and used GIS to calculate surface area.

7/27/2012LaurelPond_Tables_Figures_v2_tw

Average Discharge

Annual Discharge

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec (mgd) (acre-ft)

1985 1.20 1.18 1.29 1.03 0.82 1.10 1.511986 1.76 1.74 2.00 1.85 1.43 1.11 1.14 1.16 0.90 1.15 1.11 1.29 1.39 1,5601987 1.68 1.75 1.80 1.58 1.23 1.24 1.28 1.38 1.12 1.01 1.15 1.63 1.40 1,5711988 1.84 1.89 1.66 1.38 1.37 1.37 1.47 1.47 1.29 0.98 1.07 1.48 1.44 1,6171989 1.72 1.90 1.78 1.40 1.27 1.30 1.52 1.56 1.35 1.11 1.08 1.31 1.44 1,6121990 1.45 1.46 1.46 1.35 1.10 1.27 1.43 1.43 1.28 1.04 0.99 1.06 1.28 1,4281991 1.17 0.89 1.01 1.05 0.96 0.85 1.07 1.07 0.96 0.74 0.82 1.08 0.97 1,0901992 1.35 1.35 1.30 1.09 0.88 0.90 1.16 1.18 1.02 0.87 0.81 1.10 1.08 1,2241993 1.34 1.35 1.32 1.42 1.52 1.21 1.25 1.24 0.99 0.85 0.78 1.06 1.19 1,3371994 1.50 1.10 1.12 1.00 1.33 1.21 1.37 1.37 1.11 0.96 0.96 1.30 1.19 1,3391995 1.34 1.30 1.40 1.49 1.84 1.55 1.77 1.38 1.11 0.96 0.85 1.26 1.35 1,5261996 1.37 1.55 1.44 1.63 1.59 1.30 1.42 1.52 1.21 0.97 0.97 1.45 1.37 1,5371997 1.87 1.42 1.64 1.48 1.22 1.14 1.40 1.50 1.09 1.01 0.98 1.35 1.34 1,5041998 1.52 1.35 1.47 1.53 1.54 1.65 1.48 1.53 1.23 1.02 1.01 1.36 1.39 1,5701999 1.44 1.37 1.37 1.35 1.19 1.18 1.41 1.60 1.32 1.00 0.92 1.19 1.28 1,4322000 1.27 1.40 1.38 1.51 1.32 1.19 1.43 1.54 1.22 1.02 1.00 1.37 1.30 1,4662001 1.48 1.51 1.52 1.40 1.12 1.15 1.50 1.57 1.13 0.95 0.97 1.28 1.30 1,4542002 1.51 1.48 1.55 1.47 1.10 1.07 1.50 1.48 1.07 1.13 1.11 1.37 1.32 1,4632003 1.63 1.52 1.67 1.67 1.46 1.13 1.27 1.41 1.14 1.02 0.99 1.35 1.36 1,5152004 1.48 1.28 1.36 1.42 0.97 0.93 1.14 1.18 0.95 0.86 0.99 1.40 1.16 1,3072005 1.50 1.40 1.41 1.38 1.81 1.32 1.30 1.40 1.06 0.90 0.94 1.32 1.31 1,4622006 1.65 1.64 1.60 1.93 2.14 1.37 1.37 1.40 1.14 1.00 0.98 1.26 1.46 1,6312007 1.49 1.39 1.34 1.30 0.99 1.07 1.33 1.32 1.02 0.91 0.87 1.24 1.19 1,3322008 1.41 1.41 1.39 1.39 1.18 1.18 1.37 1.51 1.08 0.89 0.91 1.28 1.25 1,4032009 1.41 2.08 2.25 2.08 1.25 1.13 1.31 1.44 1.12 0.94 0.88 1.26 1.43 1,5982010 1.43 1.40 1.31 1.35 1.30 1.30 1.33 0.80 0.92 0.85 0.94 1.33 1.19 1,3312011 1.46 1.45 1.39 1.74 1.58 1.35

Table 2

YearAverage Monthly Discharge (mgd)

1Discharge to Laurel Pond began in June 1985. Bold italicized values were estimated.

MCWD Effluent Discharge to Laurel Pond From 1985 to 20111


Evaporation Elevation(in/yr) (ft)

June Lake Calculation 38 7,630 DWR (1981)Grant Lake Land pan 38 7,144 Gram/Phillips (1985)Grant Lake Floating pan; seasonal 43 7,144 Lee (1969)Laurel Pond Hybrid (Bishop/LVD) 43 7,133 WEI (2011)Long Valley Dam Land pan; seasonal 41 6,789 LADWP; Jones and Stokes (1993)Long Valley Dam Floating pan; seasonal 52 6,789 LADWP; Jones and Stokes (1993)Bridgeport Reservoir Calculation 43 6,464 Lopes and Alexander (2009)Mono Lake Calculation 48 6,384 LADWP; Jones and Stokes (1993)Bishop No. 35 Calculation 59 4,170 CIMIS (2011)

Station Name Type Source

Table 3Evaporation Estimates for Laurel Pond and Other Stations in Mono and Inyo Counties

7/27/2012LaurelPond_Tables_Figures_v2_tw.xlsx

MCWD Wastewater Discharge to Laurel Pond

Precipitation on Laurel Pond

Aggregate Natural Inflow to Laurel Pond

Total Inflow InfiltrationEvaporation From Laurel

Pond

Total Outflow

1998 174 1,570 166 972 2,708 2,398 318 2,716 -8 166

1999 168 1,432 79 364 1,874 1,592 308 1,900 -26 142

2000 144 1,466 102 295 1,863 1,529 319 1,848 15 159

2001 161 1,454 164 241 1,859 1,509 341 1,850 9 168

2002 170 1,463 105 206 1,774 1,434 311 1,745 29 197

2003 200 1,515 113 349 1,977 1,648 346 1,994 -16 183

2004 185 1,307 136 195 1,638 1,337 298 1,634 3 188

2005 190 1,462 176 914 2,552 2,181 373 2,553 -1 190

2006 204 1,631 157 1,178 2,965 2,603 395 2,999 -33 160

2007 162 1,332 72 106 1,510 1,223 297 1,520 -10 152

2008 155 1,403 128 188 1,719 1,366 338 1,704 15 167

2009 170 1,598 132 327 2,057 1,686 378 2,064 -7 162

2010 165 1,331 198 690 2,218 1,801 349 2,150 68 231

Average 173 1,459 133 463 2,055 1,716 336 2,052 3 174Ratio of

Component

to Total1na 71.0% 6.5% 22.5% 100.0% 83.6% 16.4% 100.0% na na

Maximum 204 1,631 198 1,178 2,965 2,603 395 2,999 68 231

Minimum 144 1,307 72 106 1,510 1,223 297 1,520 -33 142Standard Deviation

17 102 38 352 438 425 32 445 26 23

Coefficient of Variation

10.1% 7.0% 28.8% 75.9% 21.3% 24.8% 9.4% 21.7% 931.6% 13.3%

1Ratio is relative to total inflow for inflow terms and relative to total outflow for outflow terms.

Table 4Annual Calendar Year Laurel Pond Water Budget for the Calibration Period (1998-2010)

(acre-ft)

YearStart of

Year Storage

Inflows Outflows

Inflow-Outflow

End of Year Storage


Low Medium High Low Medium High

2015 1,670 480 480 480 1,190 1,190 1,190

2030 2,330 640 740 1,015 1,690 1,590 1,315

Table 5

(acre-ft)

Source: Table 4-5 and 4-6 from the 2010 MCWD Urban Water Management Plan and personal communication with MCWD staff on March 26, 2012.

Reuse Scenario Net Discharge to Pond

YearRecycled

Water Production

Summary of Reuse Scenarios and Net Discharge to Laurel Pond


Min Max Avg Min Max Avg Min Max Avg

2015 50 114 87 50 114 87 50 114 87

2030 70 120 99 64 120 98 47 120 94

`

Summary of Laurel Pond Areas for the 2015 and 2030 Reuse ScenariosTable 6

(acres)

Range in Pond Area

Low Reuse Scenario Medium Reuse Scenario High Reuse ScenarioYear


0 1Miles

0 1Kilometers

L O N G V A L L E Y

É

Produced for:

017-008004

Figure 1

Path:

N:\M

apDo

cs\C

lients

\MCW

D\La

urel P

ond\R

eport

\Figu

re1_L

ocati

on.m

xd

www.wildermuthenvironmental.com

Produced by:

Areal Location Map

KÏ

MCWDWaste Water Treatment Plant

LaurelPond

Mammoth/YosemiteAirport

To Mammoth Lakes

MC395

Laurel Springs

Stream Gauging Station#*

Treatment Plant Outfall")

Laurel Pond Monitoring Well@A

@A

@A

@A

@A")

Qt

Qoa

Qal

Qpb

Qoa

QpbQal

Qti

Long Valley Caldera

LP No. 3LP No. 4 LP No. 1

LP No. 2

0 500 1,000Feet

0 100 200 300MetersÉ

Produced for:

017-008004

Figure 2

Path:

N:\M

apDo

cs\C

lients

\MCW

D\La

urel P

ond\R

eport

\Figu

re2_G

eolog

y.mxd


Produced by:

Geologic Map

KÏ

Laurel Pond

Qal - Younger alluviumQoa - Older alluvium

Qpb - Porphyritic basalt

Qt - TalusKg - Granodiorite

Qti - Tioga TillPzms - Metasedimentary rocks

FaultsLocation Certain

Treatment Plant Outfall")

Laurel Pond Monitoring Well@A

Cross-Section Profile

Laurel Pond

Basalt

4/2/2012 -- 2:41 PMFlowDiagramLP.ai

Figure 4. Laurel Pond Schematic Water-Budget Model

Inflows

MCWDWastewater Precipitation

Surface water as:Spring flowStorm flowSnowmelt

Groundwater

Laurel Pond

Outflows

Evaporation Infiltration togroundwater

Figure 5

Historic Aerial Images of Laurel Pond1986, 1988, 1993, 1998, 2005, and 2010

Produced by:

23692 Birtcher DriveLake Forest, CA 92630949.420.3030www.wildermuthenvironmental.com »

0 750 1,500 2,250 3,000Feet

0 200 400 600 800Meters

004017-008

Author: MABDate: 7/24/2012

Image Date: 8/30/1986Agency: NHAPArea: 56 acres

Image Date: 08/07/1988Agency: NAPPArea: 38 acres

Image Date: 08/23/1993Agency: MCWDArea: 52 acres

Image Date: 08/26/1998Agency: USGSArea: 109 acres

Image Date: 08/26/2005Agency: NAIPArea: 85 acres

Image Date: 07/17/2010Agency: NAIPArea: 88 acres

Name: Figure5_AerialPhotos

Figure 6Laurel Pond Bathymetry

Produced by:

www.wildermuthenvironmental.com É 017-008004Name: Figure6_Bathymetry

Basemap: 2011 Aerial Bing Maps

Author: MABDate: 7/24/2012

0 200 400 600 800100Feet

0 100 20050Meters

Main Features

Shoreline 9/14/2011

0 - 11 - 22 - 33 - 44 - 55 - 5.60

Laurel Pond Depth (feet)

7/24/2012LaurelPond_Tables_Figures_v2.xlsx

7120

7122

7124

7126

7128

7130

7132

7134

7136

7138

7140050100150200250300350

0 500 1,000 1,500 2,000 2,500 3,000 3,500

Area (acres)

Elev

atio

n (ft

-msl

)

Storage Capacity (acre-feet)

Figure 7 Laurel Pond Elevation-Area-Storage Curves

Volume Area


0

200

400

600

800

1000

1200

1400

1600

180019

86

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

Volu

me

(acr

e-ft)

Year

Figure 8 Total Annual Effluent Discharge to Laurel Pond

1986 to 2010

!(

!(

!(

!(

!(

!(

BridgeportRanger Station

Long Valley DamSNARLKMMH

LaurelPond

Bishop 35

Lake Mary

Mono Lake

Lake Crowley

BridegportReservoir

GrantLake

JuneLake

ConvictLake

Bishop

Mono City

Bridgeport

Mammoth Lakes

Yosemite Village

118°W119°W

38°N

Mammoth Lakes

Produced for:

017-008004


Produced by:

Figure 9

Precipitation and EvaporationStations near Laurel Pond

Path:

N:\M

apDo

cs\C

lients

\MCW

D\La

urelP

ond\R

eport

\Figu

re9_P

recipE

TStns

.mxd

0 10 20Miles

0 10 20 30KilometersÉ

Laurel PondElevation: 7,135 ft

Main FeaturesPrecipitation Station

Precipitation and Evaporation Station

!(

!(

Precipitation and Evapotranspiration Station!(

) PRISM Precipitation 800m Grid

NV

CA

NEVADA

IÈ

Station Latitude Longitude Elevation (ft)

Observation Frequency Comments

B is hop 35 37.356 -118.403 4,170 Daily Rainfall at B is hop 35 is not repres entat ive of ra infall at Laurel P ond bas ed on P RIS M data

Bridgeport Ranger Station 38.361 -119.382 6,470 Daily Rainfall at Bridgeport Ranger Station is representative

of rainfall at Laurel Pond based on PRISM data

Lak e M ary 37.599 -118.999 9,004 M onthly Rainfall rec ords at Lak e M ary are not tem porarily repres entat ive

Long V alley 37.587 -118.699 6,800 Daily / M onthly

Rainfall rec ords at Long V alley are not tem porarily repres entat ive

K M M H 37.621 -118.834 7,129 Daily Rainfall rec ords at K M M H are not tem porarily repres entat ive

S .N.A .R.L. 37.614 -118.831 7,100 Daily Rainfall rec ords at S .N.A .R.L. are not tem porarily repres entat ive


40

50

60

70

80

90

100

110

120

130

140

1/1/98 5/16/99 9/27/00 2/9/02 6/24/03 11/5/04 3/20/06 8/2/07 12/14/08 4/28/10

Area

(acr

es)

Date

Figure 10 Laurel Pond Surface Water Model Calibration

Observed Area Simulated Area


0

20

40

60

80

100

120

140

1/1/1986 9/27/1988 6/24/1991 3/20/1994 12/14/1996 9/10/1999 6/6/2002 3/2/2005 11/27/2007 8/23/2010

Area

(acr

e)

Date

Figure 11a Laurel Pond Area Using 2015 Planned Recycling Alternatives

Planned Reuse 18 Acres


0

20

40

60

80

100

120

140

1/1/86 9/27/88 6/24/91 3/20/94 12/14/96 9/10/99 6/6/02 3/2/05 11/27/07 8/23/10

Area

(acr

e)

Date

Figure 11b Laurel Pond Area Using 2030 Planned Recycling Alternatives

Reuse Scenario Low Reuse Scenario Medium Reuse Scenario High 18 Acres

Documents

July 27, 2012 Mammoth - Mammoth Community Water District...Mammoth Lakes and approximately 2 miles southwest of the Mammoth/Yosemite Airport. Laurel Pond is located along the southern