Hydrogeological Site Investigation and Mounding Analysis for Onsite Wastewater Discharge

Blaire MacAulay, P.Geo., M.Sc .

Base l i ne Wa te r Resou rce I nc .

w w w. b a s e l i n e w a t e r . c o m

C l e a r S t a r t . C l e a n F i n i s h .

Outline Status of site investigation & mounding analysis in Alberta

Private Sewage Systems Standard of Practice - 2009 revision

2015 Updates

Site investigation process

Mounding analysis modelling

Process timeline summary

Design of long term groundwater monitoring network

Background Groundwater Mounding:

“Rise in water table height beneath a water discharge site.”

(Poeter et al., 2005)

Adverse Effects:

Insufficient vertical separation Favours anaerobic conditions

Reduces treatment effectiveness

Lateral groundwater transport

Potential contamination of aquifer

Increased risk of treatment system failure

Revision Status Disconnect between site investigation

& mounding SOP requirements and practice

2015 Revision of SOP

Plumbing Technical Council Task Group

More stringent requirements for site assessment and mounding

Expectation for site investigation & mounding unchanged since 2009

Revision Status Objective:

Clearly outline requirements for proper site investigation & mounding analysis

Result:

Assist users in planning feasibility-phase site investigation

Assist Regulators in review of application

More stringent requirements become regulations and aid in protection of receptors

2015 SOP Changes/Trends

Site Investigation & Mounding Related:

1. Updated hydrogeologic investigation and mounding requirements for systems > 9 m3/day

2. Investigation must be adjusted based on site complexity

3. Further protection of waterbodies & contaminant/pathogen attenuation

4. Increased requirements for monitoring well placement

5. Assess chemistry of domestic water supply input into system

6. Define long term monitoring and contingency plans

Site Investigation Process

Phases:

1. Initial site visit

2. Program planning

3. Field work & data collection

4. Data analysis

5. Reporting & recommendations

Objective: “…to assess and quantify the capability of the site to infiltrate and disperse the effluent load into the soil in a manner that obtains treatment objectives…” (Safety Codes Council, 2009)

Initial Site Visit (1 Day)

Purpose:

Better understand site conditions prior to planning a detailed field program.

Observe:

Vegetation

Site layout & infrastructure

Surface water, drainage areas

Topography

Access

Soil conditions (< 30 cm)

Involve project personnel from multiple disciplines

Program Planning Requirements: Locate materials & equipment

Retain contractors

Ground disturbance

Background geologic research

Review maps/prints/GIS/surveys

Coordinate mobilization

Scheduling

Safety training & gear

Site prep (clear-cutting, roads)

Logistics & packing

Communication

Weather considerations

Sample handling

Accommodations & meals

Budgeting & cost

Etc….

Hydrogeologic Investigation Design:

Define objectives & scope

Location of monitoring wells/boreholes/pits

Sample collection procedures

Select appropriate drilling method

Design monitoring well completions

Lab analytical parameters

Meet requirements of SOP A lot of prep work!

Fieldwork & Data Collection

Requirements:

2009 SOP: Detailed site investigation required for all systems > 5.7 m3/day

2015 Revision: Most notable changes for Hydrogeologic Characterization

1) Soil Characterization 2) Hydrogeologic Characterization 3) Detailed Site Description

1) Soil Characterization

Investigation Depth (2015):

Small (5.7 – 9 m3/day)

1. >300 mm (1 ft) below depth necessary to prove vertical separation, AND

2. Determine effluent linear loading capacity

Prescribed depths from 2009 SOP removed

Minimum Number of Profiles (2015):

Two locations: at least 1 excavated test pit. Other 1 borehole with intact soil coring

Additional locations for complex site, or if system area increases

Investigation Depth (2015):

Large (>9 m3/day)

1. Maximum depth of 15 m (50 ft)

Soil Sampling

Soil Sampling:

Frequency: 1 sample per 0.5 m depth

Representative sample of each distinct soil unit

Grab samples in bags or jars

Basic characterization

Grain size analysis

Collect intact soil cores

Observe soil structure

Used for lab hydraulic conductivity testing

Soils must be continuously logged during drilling, or within test pit.

Soil Classification

Canadian System of Soil Classification

Grain Size Analysis: Hydrometer: Sand/Silt/Clay

Sieve: Sand breakdown Fine/Medium/Coarse

Note:

Colour

Grain size

Mottling

Structure

Consistency

Roots

Pores

Clay films

Concretions

Precipitates

Coarse fragments

Horizon boundary

Unit thickness

Reaction to acid

Soil Analytical Parameters

Establish baseline soil chemistry.

Laboratory Analyses:

Electrical Conductivity (EC)

Sodium Adsorption Ratio (SAR)

pH

Detailed Salinity

Metals

Grain Size

Soil EC Probe

(Field)

Soil Investigation Objective Summary

Deliverables:

1. Continuous, detailed soil profile logs

Identify coarse vs. fine soils

Identify structures that limit or enable flow

2. Depth to limiting soil layers

Clays, very fine sediment

3. Background/baseline soil chemistry

4. Variation in site soil properties

Heterogeneity (vertical and horizontal)

2) Hydrogeological Investigation

Investigation Depth:

Maximum depth of 15 m (50 ft)

Justification from hydrogeological professional

Minimum Number Monitoring Points:

Three groundwater monitoring wells

Additional locations for complex site, or if system area increases

2015 SOP Revision Changes:

Additional site evaluation for large (>9 m3/day) systems

Monitoring Well Placement

Layout reflects:

Size & shape of discharge system

Direction of regional groundwater flow

Geology & hydrogeology

Conceptual Monitoring Well Network

Position wells:

Upgradient of system (background)

Downgradient of system

Cross gradient

Between system and waterbody

Between system and water source well

Between system and adjacent buried infrastructure

Drain Field

Monitoring Wells

Groundwater Flow Direction

Lake

Water Source Well

Buried Sand Channel

Monitoring Well Completions

Pre-pack Screen

Steel Casing

2” Solid PVC

Lock

Bentonite Chips

Filter Sand

Wastewater Discharge

Monitoring Well Completions

Sand

Sand

Silty Clay

Clay

Monitoring wells completed at different depths

Requirement of 2015 SOP > 9 m3/day

Groundwater Flow Regime

Based on water table surface elevation

Hydraulic Conductivity (K) *Paramount for mounding analysis!

Slug test measures horizontal hydraulic conductivity of saturated zone.

Bouwer-Rice Model: Calculate K

Groundwater Analytical Parameters

Laboratory Analyses:

Routine Parameters Chloride, hardness, nitrogen, pH, alkalinity, ions

Total & Dissolved Metals

Biological Oxygen Demand (BOD)

Chemical Oxygen Demand (COD)

Bacteriological Parameters

Fecal Coliform & E. coli. bacteria

Nutrient Parameters Phosphorous, Nitrogen, Dissolved Organic Carbon

(DOC), Total Kjeldahl Nitrogen, Ammonia, etc…

Other site-specific parameters

Establish baseline groundwater chemistry

Sample collected after monitoring well development

Transport & Attenuation Modelling

2015 SOP: For systems > 9 m3/day

Guidance referenced in 2015 SOP

Required when:

Potential for affected groundwater to reach a river/lake/stream/creek

Waterbody within 1 km of system

Purpose:

Estimate loading potential of phosphorous, nitrogen, and chloride to surface water

(Gerrard, 2010)

Hydrogeological Investigation Objectives

Deliverables:

1. Static water level elevation Available vertical separation

2. Direction of site groundwater flow

3. Hydrogeologic setting

4. Horizontal hydraulic conductivity of saturated zone (field)

5. Vertical hydraulic conductivity (lab)

6. Baseline groundwater chemistry

7. Transport & attenuation modelling (if near waterbody)

8. Site wide heterogeneity

3) Detailed Site Description

Bedrock

Topography

Surface Water

Vegetation - Dry

Flood Plain

Weather Conditions

Infrastructure

Mass Wasting

Mounding Analysis

Required when:

1. The available vertical separation distance to a limiting soil layer does not exceed 300 mm (1 foot); OR

2. Daily peak flow exceeds 5.7 m3 (2009)

3. Daily peak flow exceeds 9.0 m3 (2015)

2015 Revision, >9 m3/day:

Complexity of analysis required must reflect complexity of system

Based on design flow volume

Based on results of site investigation

Mounding Analysis

Calculate Mounding Height: Hantush 1967 Analytical Solution

Mounding Height (zmax) Function of:

Loading Rate (q’)

Initial saturated thickness (hi) of aquifer

Horizontal hydraulic conductivity (Kh)

Discharge area (l, w)

Time since infiltration began (t)

Specific yield (Sy) Literature sourced value

Hantush solution highly dependent on hydraulic conductivity (K)

Accurate field measure of K is a must!

Mounding Principles

Mound rises from static water level up.

Mound necessary to drive groundwater away from site.

Goal is to achieve balance between input and output rate away from site.

Steady State.

(Modified after Carleton, 2010)

Mounding Analysis

Analytical Model: “First Step”

Mound height at center of

drainfield

Groundwater breakout on

side slope

Poeter et al. 2005

Model Assumptions & Limitations

1. Aquifer is homogeneous & isotropic

2. Aquifer extent is infinite

3. Static water level is horizontal No regional flow

4. Depth to limiting soil layer is constant

5. Infiltration is vertical until water table No vadose zone spreading

6. No boundary condition effects

Model complexity should match site complexity and risk.

Numeric models allow for more complexity (i.e. MODFLOW)

Higher cost, more detailed site investigation required

Requires professional interpretation

Simplification of Site

Misconception: Mound Growth Mound height growth is logarithmic.

Discharging water for half the time ≠ twice the capacity

After 1 year, achieved 53% of total growth (10 years)

Potential for system failure realized after 2-3 years of system function

10.0, 8.4

0

1

2

3

4

5

6

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8

9

0 1 2 3 4 5 6 7 8 9 10

Mo

un

d H

eigh

t (m

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Years

Peak Mound Height Growth

Mounding Trend

Misconception: Drainfield Size

1. Drainfield shape

Area = 10,000 m2 Rectangle: 200 x 50 m Square: 100 x 100 m

Rectangle: 20.0 m3/day Square: 17.9 m3/day

2. Doubling drainfield area does not double capacity

Area 1: 100 x 50 = 5,000 m2 Area 2: 141 x 71 = 10,011 m2

Area 1: 15.8 m3/day Area 2: 18.5 m3/day

Misconception:

“All sites are suitable with good system design”

A maximum capacity discharge rate exists

Based on soil & hydrogeological conditions

Changing system design will not increase this maximum

Not all sites set up for success of high volume demand

More beneficial to construct multiple systems with large spacing

Consider interference effects

Process Timeline

Activity Duration

1) Planning & Site Visit 2-3 Weeks

2) Field Program 1 Week

3) Lab Results (Turnaround) 1 Week

4) Analysis, Interpretation & Modelling 3 Weeks

5) Reporting, Maps, Tables 3 Weeks

6) Design Revision 4 Weeks

7) Approval Variable

TOTAL 15 Weeks 4 Months

Long-Term Groundwater Monitoring

Purpose:

1. Operational system monitoring

2. Establish long-term trends Seasonal variation

Monitoring Well Placement:

Site investigation wells may be used long term

Add wells based on site investigation and design changes

Minimum three wells

Frequency:

Quarterly for first year

Adjusted based on baseline conditions & initial measurements

Concluding Remarks

Intent of 2015 SOP revision to facilitate design of a site investigation & mounding analysis

Improved guidelines for users and Regulators

Quality site investigation improves mounding model output

Investment in feasibility-phase site investigation & mounding reduces economic and environmental risk

Not all sites are destined for success

Water stewardship involves ongoing cooperation of all project stakeholders

Questions?

References Bouwer, H., Rice, R.C. 1976. A Slug Test for Determining Hydraulic Conductivity of Unconfined Aquifers With Completely or Partially Penetrating Wells. Water Resources Research, vol. 12, no. 3, pp. 423-428. Carleton, G.B. 2010. Simulation of Groundwater Mounding Beneath Hypothetical Stormwater Infiltration Basins. U.S. Geological Survey Scientific Investigations Report 2010-5102, pp. 64 Gerrard, J. 2010. Diane Orihel – and a new approach to algal blooms on Killarney Lake – and the implications for Lake Winnipeg?. October 6, 2010. Available online at: http://manitobaliberals.blogspot.ca/2010_10_03_archive.html. Accessed March 2, 2015. Hantush, M.S. 1967. Growth and Decay of Groundwater Mounds in Response to Uniform Percolation. Water Resources Research, Vol. 3, pp. 227-234. Poeter, E., McCray, J., Thyne, G., Siegrist, R. 2005. Guidance for Evaluation of Potential Groundwater Mounding Associated with Cluster and High-Density Wastewater Soil Absorption Systems. Project No. WU-HT-02-45. Prepared for the National Decentralized Water Resources Capacity Development Project, Washington University, St. Louis, MO, by the International Groundwater Modeling Center, Colorado School of Mines, Golden, CO. Safety Codes Council. 2012. Alberta Private Sewage System Standard of Practice Handbook. Edmonton, Alberta.