EVALUATION OF GROUNDWATER FLOW AND …Alkalinity was measured in situ using the Hach® alkalinity digital titrator, model 16900 using 0.5 N Na 2CO 3 standard. Titration with 0.16 N

Thirteenth International Water Technology Conference, IWTC 13 2009, Hurghada, Egypt

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EVALUATION OF GROUNDWATER FLOW AND TRAVEL TIMES

USING 14C

U. A. Abu Risha*1, I. Clark1, and S. Beecham1,2

1 The University of South Australia, Australia 2 Centre for Water Management and Reuse, Australia

*Corresponding Author, NBE, UniSA, Mawson Lakes Boulevard, Mawson Lakes, 5095; E-mail: [email protected]

ABSTRACT Evaluating groundwater flow is important for managing groundwater resources. This can be done using an isotopic tracer such as 14C in conjunction with hydraulic data. The Dalhousie Springs represent one of the main natural discharge points in the Great Artesian Basin (GAB). The high 14C activity of these springs compared to those of the uppergradient wells implies local mixing with younger groundwater. Upgradient of the Dalhousie Springs, the system can be described by a linear piston flow model. According to this model the recharge rates range from 0.64 to 1.07 mm/year whereas the groundwater flow velocity is about 3.8 m/year. Keywords: 14C dating, Flow velocity, Mixing, Netpath 1. INTRODUCTION AND GENERAL BACKGROUND 1.1 Study area The famous Dalhousie Springs represent the most important natural discharge points in the Great Artesian Basin (GAB) (Figure 1). Although the area is typically arid with annual rainfall average of about 130-200 mm/year (Figure 2) and evaporation rate exceeding rainfall by an order of magnitude or more throughout the year (Allen [1]), occasional flash flooding tends to occur especially in late summer monsoon. Floods of magnitudes up to 200 mm/d were recorded from Alice Springs (Figure 2b) and Oodnadatta. Regional recharge to the GAB occurs via its peripheral outcrops. The GAB aquifers at Dalhousie are hosted in the lower Cretaceous Algebuckina Sandstone and the Cadna-owie Formation. Recharge to the Dalhousie Springs is from the GAB western recharge area (Fig. 1). This part of the GAB represents a separate flow system that is distinct from the other GAB flow systems (Habermehl [2]; Torgerson et al. [3]). The salinity of the non-artesian groundwater in the shallow Quaternary aquifers at Dalhousie varies greatly from 317 mg/l in Oasis Bore, which occurs in sand dune area, to more than 17000 mg/l in a well occurring near salt pans to the south of Dalhousie (Herraman [4]).

Thirteenth International Water Technology Conference, IWTC 13 2009, Hurghada, Egypt 1350

Figure 2 (a) Annual rainfall average (mm) in Central Australia; (b) Maximum daily rainfall and their dates, Alice Springs (Commonwealth of Australia [7])


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1.2 Carbon-14 as a groundwater dating tool Carbon-14 (half life = 5730 years) is produced in the atmosphere by the bombardment of 14N by neutrons. The produced 14C oxidizes into 14CO2 and mixes with the atmospheric gases. Neutron fluxes in the subsurface from the spontaneous fission of U and other radionuclides can produce hypogenic 14C by neutron activation of 14N or neutron capture by 17O and � decay. However, the subsurface 14C production is negligible compared to the other subsurface 14C dilution sources (Clark and Fritz [8]). 14C activity can be measured in both groundwater inorganic and organic carbon. Carbon-14 ages can be calculated using

t = -1/� x ln(14Cs/14Cr) (1) where � is the 14C decay constant (� = 121 x 10-6) and the factor 1/� is the 14C mean life (8267 years), 14Cs is the 14C activity in the groundwater sample in percent modern carbon (PMC), and 14Cr is the 14C activity in the recharge water in PMC as well. Despite the complex chemistry of carbonates in groundwater, 14C is widely used in dating groundwaters recharged between 500 and 30 000 BP. This advantage encouraged scientists to search for models to correct the measured 14C activities for the subsurface carbonate reactions that may dilute the initial 14C input in the recharge water (Vogel [9] & 10], Ingerson and Pearson [11], Tamers [12, 13 & 14], Mook [15 & 16], Fontes and Garnier [17], Plummer et al. [18], Eichinger [19]). In carbonate aquifers, the dilution effects on the 14C ages are severe. Contrarily, in granitic and sandstone aquifers composing mainly of quartz and weathering-resistant feldspars the dilution effects are expected to be insignificant (Clark and Fritz [8]). 1.3 Aim and Scope The aim of this work is to focus on the importance of combining the hydrochemical and hydrogeologic techniques in studying regional groundwater flow systems. This approach has been applied successfully in different hydrogeologic environments (e.g. Love et al. [20], Zhu [21], Harrington et al. [22]). This work presents a situation in which the local spatial distribution of 14C reflects flow direction that is locally opposite to the regional flow direction due to receiving modern recharge or mixing with younger groundwater. This pseudo-direction may lead to wrong management and protection decisions of the Dalhousie Springs. The impact of mixing on groundwater ages using radioactive tracers may be severe (Bethke & Johnson [23]). Up to date the local groundwater flow in the area of the Dalhousie Springs has not been fully understood. It’s been suggested that the springs receive their recharge from the Finke River (Figure 1) where the recharged floodwaters flow along NNE faults and fractures towards the Dalhousie Springs (Radke et al. [5]). However, other scenarios are also possible. Based on the groundwater 14C, two of these scenarios are discussed in this paper.


2. METHODOLOGY 2.1 Sampling Six springs, two artesian wells, and one pumped well were sampled and analyzed for major ions, 13C, and 14C in order to investigate the groundwater flow to the Dalhousie Springs. Major ion samples were filtered through 0.45-µm membrane filters. In addition, the cation samples were acidified by nitric acid to pH<2 in situ. Samples for 13C and 14C analyses were collected in 5 liter heavy plastic containers. Trace amounts of Na-azide were put in the empty containers before sampling to stop any biologic activity (Clark and Fritz [8]). Sampling from the spring pools was performed using an inflatable rubber boat and a 12 V electrical DC powered pump. Samples were collected as close as possible to the spring vents. The non artesian (Mt. Dare) well was operating for more than 2 hours when the sample was taken. No information about the design of this well is available. The shut-in well (3O’Clock) was let to flow for about 15 minutes before sampling it. The sample from the controlled flowing well (Junction) was taken from its flowing tube. The screen of this well is about 40 m length. Alkalinity was measured in situ using the Hach® alkalinity digital titrator, model 16900 using 0.5 N Na2CO3 standard. Titration with 0.16 N H2SO4 continued to pH 4.5. A multiparameter thru-flow cell was used to measure the groundwater electrical conductivity (EC), temperature, pH, Eh, and DO directly from the source. EC, pH, temperature, DO, and pressure profiles were measured with depth in the spring pools using YSI® 600XL Series Sonde. Three rock samples taken from different depths in the slotted interval of the Junction Bore were analyzed for their major ion concentrations and mineral compositions.

2.2 Analytical techniques All analyses including the preparation of 14C samples were conducted at the CSIRO Land and Water, Adelaide Analytical Laboratories. Only measuring the 14C activities were conducted at Australia National University (ANU) using the accelerator mass spectrometry (AMS). Major and minor cations were analysed by ICP-OES with a precision of < 5%. All anions were analysed by ion chromatography with precision of about 4%. The 14C and 13C samples were extracted from the dissolved inorganic carbon (DIC) as CO2. Carbon-13 was measured on an aliquot taken from some of this CO2. Conducting 13C analysis requires 10 mg C. The precision of the 13C measurements is 0.2 %. Another amount of the extracted CO2 was put into a tube with iron powder serving as a catalyst for the carbon to attach to. The mixture was heated with hydrogen to 570 ºC to produce elemental carbon and water. The elemental carbon was put into a sample holder and pressed into little pellet. The prepared pellets were loaded into the AMS. Performing the analysis requires about 1 mg of carbon. Results are expressed in percent modern carbon (PMC). The major ion concentrations in the aquifer rocks were measured by X-ray fluorescence (XRF) whereas the bulk mineralogy of the aquifer rocks was quantitatively estimated by X-ray diffraction (XRD).


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2.3 Data Handling All the groundwater chemistry and isotope data were stored in EXCEL spreadsheet. Then, a “.pat” file was created using the USGS NetpathXL program (Plummer et al [24]; Parkhurst and Charlton [25]). The minerals detected by the XRD were used as reactive phases. Some of the chemical elements that known to exist in these phases were selected as constraints of the chemical reaction models. The samples were dated assuming that the 14C activity in the soil gas CO2 is 100 PMC whereas the carbonate 14C is 0 PMC. The carbonate13C was assumed to be 0 ‰. 3. RESULTS The major ion concentrations and isotopic compositions of the studied samples are given in Table 1. The aquifer rocks are composed of quartz (95 %), microcline (1.6 %), and kaolinite (3.3 %). This composition reflects the strong weathering-resistant nature of the aquifer and implies that the water-rock interactions are too slow. Consequently, the main factors affecting the groundwater evolution are expected to be (a) the initial composition of the recharge water at the time it reached the water table, which depends on the climatic conditions and the composition of the soil and unsaturated zones; and (b) mixing with other groundwater plumes from the GAB or other aquifers having different chemical compositions. The NETPATH models show that the cation exchange of Ca for Na, the dissolution of microcline, and the simultaneous precipitation of kaolinite and calcite are the main chemical processes in the aquifer. The modelled 14C ages of the GAB wells are given in Table 2. These ages agree with the hydrologic setting of the GAB in this region (Figure 1). The ages of the Mt. Dare, Junction, and 3O’Clock wells are about 19, 24, and 23 ka, respectively. According to the estimated 14C ages the groundwater velocity along the flow path extending from the Claude well to the Mt, Dare well (Figure 1) is about 3.8 m/year which is close to the velocity estimated previously for the same flow path (Radke et al. [5]). Although the Dalhousie Springs have physical, chemical and isotopic signatures quite similar to those of the upgradient 3O’Clock well, their 14C activity is higher reflecting mixing of the GAB groundwater with younger water. The temporal variations of the conductivity and flow rate of the Dalhousie Springs reflect the occurrence of seasonal recharge to these springs (Figure 3). The abrupt spring pool water conductivity increase and temperature decrease at specific depth reflect the occurrence of discharge of shallow saline cold water into the spring pools (Figure 4). The source of this shallow water is expected to be recent local recharge. This process has resulted in abrupt decrease of the ages of the Dalhousie Springs’ samples. Using NETPATH, the decrease was found to range from about 5 ka (Dha3) to 14.5 ka (Daa5).


Table 1 Major ion concentrations (mg/l), 13C (‰ PDB) and 14C activities (PMC) in the studied groundwaters

Sample T (°°°°C) pH Ca Mg Na K Cl SO4 HCO3 13C 14C Beer Street 1 26 7 80 37 279 13 434 173 201 - 41.2 Finke Town 27 6.8 47 12 82 6 122 51.6 136 - 95.5 Sandy 1 28 6.9 43 15 92 6 146 66.8 108 -9.9 90.6 Claude 1 27 7.8 50 23 122 8 195 96.3 128 -9.1 75.3 Mt. Dare 41 6.9 28 10 250 9 278 128 123 - 5.2±0.1 Daa12 32 7.8 44 19 184 12 249 107 130 -9.5 8.1 (1.3) Daa5 34 7.2 46 19 180 11 243 112 115 -11 12.3±0.1 Dca1 39 7.7 60 23 273 14 380 158 115 -11 7.3 ±0.1 Dcd1 38 7.9 51 25 216 12 311 135 108 -10 8.2 ±0.1 Ddb3 34 7.5 46 20 241 13 315 132 129 -12 8.3±0.1 Dea1 29 7.7 46 22 241 14 311 128 148 -11 6.5±0.1 Dha3 35 7.3 61 32 259 16 384 176 111 -12 5.3±0.1 3O’clock 40 7.3 57 27 225 14 325 155 107 -11 3.1 ±0.1 Junction 46 7.4 63 34 304 19 424 197 147 -12 3.7 ±0.1 1 Recharge water (Radek et al [5]) 2 Dalhousie Spring (Radek et al [5])

Table 2 14C-modeled Groundwater Travel Times

Sample Original Data CMB Tamers Ingerson and

Pearson Fontes and

Garnier Eichinger

Mt. Dare 23622 19653 20243 19653 19615 18790 3O’Clock 27894 23926 24515 23926 23887 23062 Junction 26551 23286 23753 23286 23256 22616

Figure 3 (a) Temporary variations of the electrical conductivities of the Dalhousie Springs. (b) Temporary variations of the flow rates of the Dalhousie Springs and their relation to precipitation.


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The Dalhousie Springs 14C spatial distribution show that the GAB water mixes with younger water of variable salinity. There are two possible scenario of this process. The first is that mixing with fresh local recharge water occurs in the northeastern springs (Figure 5). Then this mixture migrates south southwestwards along shallow NNE faults and fractures. In this arid region, evapotranspiration (ET) may affect this mixture during its flow (Figure 5a). The second scenario assumes that mixing occurs locally with shallow water of different salinities (Figure 5b). In summary, the springs that have salinities higher than the 3O’Clock well salinity are a mixture of the GAB water with more saline young water whereas the springs with lower salinities are assumed to be mixtures of the GAB water with fresh young water. Unfortunately, there is no available data about the composition of the young water end member.

Figure 4 (a) The electrical conductivity profiles measured in the pools of the Dalhousie Springs show mixing of the GAB water with saltier shallow water at 4.5-5.5 m depth in the pools. (b) The water temperature profiles measured in the pools of the Dalhousie Springs show mixing of the GAB water with colder shallow water at 4.5-5.5 m depth in the pools.


Figure 5 (a) Mixing of GAB water with fresh younger water with subsequent evaporation during its flow and SSW along shallow faults. (b) Mixing of GAB water with younger waters of different salinities.

4. DISCUSSION 4.1 Carbon chemistry in the Great Artesian Basin (GAB) The total carbon content in the Junction Well production zone rocks ranges from 0.06 to 1.09 % by weight. The highest concentration was recorded from beds containing coal seams. Calcium has also very low concentrations ranging from 0.026 to 0.07 % by weight whereas magnesium concentrations range from 0.02 to 0.05 % by weight. The calcite content in the aquifer is below the detection limit. Accordingly, the carbonate reactions and, subsequently, the initial 14C dilution are not expected to be effective. The comparison between the initial 14C, as estimated by the various 14C evolution models, and the final 14C, ignoring the radioactive decay effect, shows quite slight 14C variations ranging from about 0.01 to 0.04 PMC (Table 3, Appendix A).

Table 3 Comparison between the initial and final 14C activities, ignoring decay, along with the modelled groundwater 14C ages

Well cMt. dare c3O’Clock dJunction

Model aA0 bA0nd Age

(year) A0 A0nd Age

(year) A0 A0nd Age

(year) Original Data 90.6 90.58 23622 90.6 90.54 27894 95.5 95.45 26869 Mass balance 56.06 56.04 19653 56.06 56.02 23926 61.11 61.08 23178 Tamers 60.2 60.18 20243 60.2 60.16 24515 61.44 61.41 23222 Ingerson and Pearson 56.06 56.04 19653 56.06 56.02 23926 61.11 61.08 23178 Fontes and Garnier 55.79 55.78 19615 55.79 55.75 23887 61.09 61.06 23175 Echinger 50.5 50.49 18790 50.5 50.46 23062 56.98 56.95 22599 a Initial 14C activity b Final 14C activity ignoring decay c initial 14C was measured in the Sandy Well d initial 14C was measured in the Finke Town-7 Well


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The strong agreement between the computed and the measured 13C concentrations in the final waters (Table 4) supports the 14C dating assumptions. These models assume that the soil-gas CO2

14C activity is 100 PMC whereas the modelled 13C of CO2 gas for the initial wells Sandy and Finke Town-7 are -17.661 and -19.801 ‰ PDB, respectively. C-13 is expected to increase downgradient in groundwater aquifers as a result of the dissolution of 13C-enriched carbonates. However, in sandstone aquifers composed mainly of quartz both 13C and the dissolved inorganic carbon (DIC) are not expected to evolve much beyond the conditions established in the soil (Clark and Fritz [8]). Indeed, this is the case in the studied part of the GAB where the artesian water at Dalhousie has 13C concentrations fitting well within the 13C range of the groundwater at the recharge area (Table 1). The 13C concentrations of all the studied water range from 9.1 to 12.1 ‰ PDB. This quite narrow range reflects the insignificant role of the carbonate reactions in the GAB. This outcome agrees with other studies conducted in other parts of the GAB (e.g., Herczeg et al. [26]; Radke et al. [5]). The lack of any systematic evolution of the groundwater alkalinity, DIC and 13C (Figure 6) reflects the possibility that the minor differences in these parameters are inherited from the syn-recharge reactions in the soil and unsaturated zones.

Table 4 Comparison between the final observed and modelled 13C

Well Observed 13C Computed 13C Mt. Dare -10.9 -10.0172 3O’Clock -11 -10.2506 Junction -12 -12.3615

4.2 The flow models and recharge rates The studied part of the GAB is unconfined in the recharge area and becomes confined downgradient. Assuming that the aquifer thickness increases linearly in the recharge area and that the confined part has constant thickness, the groundwater motion in the aquifer can be described by a linear piston flow model (Cook and BÖhlke [27]). According to this model, the groundwater mean transit time (T) is given by T = H�/R[(x*/x+0.5)] (2) where H is the depth of the bottom of the aquifer, � is the aquifer porosity, R is the groundwater recharge rate, x* is the horizontal distance between the confined/unconfined boundary and the sampling site, and x is the horizontal distance between the groundwater flow divide and the confined/unconfined boundary (Figure 7). The horizontal velocity according to this model is given by: V = Rx/H� (3)


Figure 6 The relationships between the DIC and alkalinity (a) and 13C (b) do not reflect any systematic evolution of these parameters as the groundwater flows downgradient in the GAB.

The model has been applied to the 3O’Clock and Junction Wells (Table 5). It is assumed that their 14C ages are 24 ka and 23 ka, respectively. The majority of the 14C models have given them ages close to these values (Table 4). The calculations assume that the GAB porosity is 10 %. The estimated recharge rates are close to the recharge rates calculated from the chloride mass balance (CMB) technique using the late Pleistocene 36Cl deposition rate (Abu Risha [28]).

Figure 7 Idealised representation of groundwater flow in confined aquifer with thickness increasing linearly with distance in recharge area and constant thickness in the confined part. Solid lines connect points of equal age whereas the broken line indicates groundwater flow path (Cook and Böhlke [27]).


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Table 5 Input data for the linear piston flow model and the resultant recharge rates and

velocities. Also shown the recharge rates calculated from the CMB

Well Age (year)

x* (km)

x (km) H (m) R (mm/year) CMB-R

(mm/year) V

(m/year) 3O’Clock 24000 3 175 300 0.65 0.49 3.77 Junction 23000 3 170 475 1.07 0.32 3.83 5. CONCLUSIONS AND RECOMMENDATIONS In sandstone aquifers, the scale of mineral-solution interactions is generally limited. These reactions are not expected to affect the initial 14C activity in groundwater. Even the subtle hydrochemical differences in these aquifers are more likely to be inherited from the reactions in the soil and unsaturated zones. So that 14C represents a very effective tool in studying sandstone aquifers. However, mixing is expected to dominate the hydrochemical evolution in these aquifers. This is evident in the increase of the 14C activities of the Dalhousie Springs due to mixing with younger water. This process has resulted in potential decrease in the groundwater ages. In such conditions relying on the 14C spatial distribution to determine groundwater flow directions is quite misleading. Consequently, it is highly recommended to use the hydrochemical and isotopic tracers in conjunction with hydraulic data. Unlike studying aquifers on the basis of well data, using springs in hydrogeologic studies is challenging. This can be attributed to the need of a more comprehensive data to understand their structure and flow mechanism. Despite the great efforts that has been done by the Australian Government to study and protect the Dalhousie Springs, the lack of detailed structural study of these springs as well as the lack of detailed investigations of the shallow and regolith aquifers in their close vicinity imposes limitations on studying the impact of mixing on their water properties. Acknowledgements This work represents a part of a PhD sponsored by the Egyptian Government to it many thanks are due. The authors are deeply gratitude to the University of South Australia (UniSA) for funding the collection and analyses of the groundwater and rock samples. Thanks are also due to the Department of the Water, Land, and Biodiversity Conservation (DWLBC) for providing the spring flow and vertical conductivity and temperature profile data of the spring pools.


Appendix A. Output File of dating groundwater by NETPATH using 14C

Initial Well : Sandy Final Well : Mt. Dare Final Initial SI 0.1479 0.3048 K 0.2359 0.1553 C 2.0963 2.2264 CA 0.7034 1.0609 K-SPAR K 1.0000 AL 1.0000 SI 3.0000 KAOLINIT AL 2.0000 SI 2.0000 EXCHANGE CA -1.0000 NA 2.0000 MG 0.0000 CALCITE CA 1.0000 C 1.0000 RS 4.0000 I1 0.0000 I2 0.0000 1 model checked 1 model found MODEL 1 Main chemical reactions K-SPAR + 0.08060 (Dissolution of microcline) KAOLINIT - -0.19935 (Precipitation of kaolinite) EXCHANGE 0.22739 (Exchange of Ca for Na) CALCITE -0.13009 (precipitation of calcite) Computed Observed Carbon-13 -10.0172 -10.9000 C-14 (% mod) 90.5786* 5.2000 Sulfur-34 0.0000 15.1000 Strontium-87 0.000000 0.710008 Nitrogen-15 0.0000 Undefined -------------------------------------------------------------------- Adjusted C-14 age in years: 23622.* * = based on Original Data Model A0 Computed Observed age (for initial A0) (initial) (no decay) (final) ------------------------------------------------------------ Original Data 90.60 90.58 5.20 23622. Mass Balance 56.06 56.04 5.20 19653. Vogel 85.00 84.98 5.20 23095. Tamers 60.20 60.18 5.20 20243. Ingerson and Pearson 56.06 56.04 5.20 19653. Fontes and Garnier 55.79 55.78 5.20 19615. F-G K -4.41 Eichinger 50.50 50.49 5.20 18790. User-defined 50.00 49.99 5.20 18708. Data used for Carbon-13 Initial Value: -9.9000000 Modeled Final Value: -10.0172011 1 precipitating phases: Average Phase Delta C Fractionation factor Isotopic composition (o/oo) CALCITE -0.13009 1.9662 -8.0114316 Data used for C-14 (% mod) Initial Value: 90.6000000 Modeled Final Value: 90.5785521 1 precipitating phases: Average Phase Delta C Fractionation factor Isotopic composition (% modern) CALCITE -0.13009 3.9323 90.9456099


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16. Mook, W. G., The dissolution-exchange model for dating groundwater with 14C, in Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology, pp. 213–225, Int. At. Energy Agency, Vienna, 1976.

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28. Abu Risha, U. A., Recharge and evolution of the artesian water at Dalhousie, South Australia. PhD thesis. The University of South Australia. In preparation.

Documents

EVALUATION OF GROUNDWATER FLOW AND …Alkalinity was measured in situ using the Hach® alkalinity digital titrator, model 16900 using 0.5 N Na 2CO 3 standard. Titration with 0.16 N