Comparison of Surface- Groundwater Interaction Model Approaches to Establish Surface Water Protection Zones in Stanislaus County

Transcription

1 Comparison of Surface- Groundwater Interaction Model Approaches to Establish Surface Water Protection Zones in Stanislaus County Bob Abrams, Ph.D., PG, CHG, Jacobson James & Associates Mike Tietze, PG, CEG, CHG, Jacobson James & Associates Jeff Harvey, Ph.D., Harvey Consulting Group 1

2 Overview Background and Key Concepts Stanislaus and Tuolumne River Surface Water depletion studies for implementation of a County Groundwater Ordinance Three approaches: One-dimensional analytical model Three-dimensional analytical element model Three-dimensional numerical model 2

3 One Resource? Response time: years to decades Water supply Dry weather storage buffer GROUNDWATER Drawdown and subsidence Water rights Regulations 3 SURFACE WATER Response time: Surface/ days to months Groundwater Water supply Interaction Groundwater Dependent Ecosystems Reservoir storage Flood control and mandated flows Water rights Conjunctive Regulations Use Riparian/aquatic ecosystems

4 Key Concepts Safe Yield Sustainable Yield Water Rights Long-Term Reliability Economical Access Societal Economic Ecological Alley & Leake (2004): Water resources cannot be developed without altering the natural environment; thus, one should not define basin yields, either as safe or sustainable, without carefully explaining the assumptions that have been made about the acceptable effects of ground water development on the environment. 4

5 Key Concepts Groundwater Extraction Sustainable Yield Intercepted Discharge or Captured Recharge Natural Recharge Pictures taken from Water Supply Protection for Rural Communities in Washington State, by Horsley Witten ( 5

6 Surface Water Depletion Controlling factors: Aquifer characteristics (especially Diffusivity) Distance between well and surface water Depth of well below surface water (similar to distance) Proximity to and variability of other recharge sources Common Misconceptions: Depends on whether a stream is gaining or losing Depends on groundwater flow direction Stops when pumping ceases Is eliminated by pumping below aquitards

7 Stanislaus & Tuolumne Rivers In 2014, Stanislaus County adopted a groundwater ordinance aligned with California s Sustainable Groundwater Management Act (SGMA). New wells must be demonstrated not to cause surface water depletion that has a significant and unreasonable impact on surface water resources. A modeling evaluation was conducted to establish a well permitting procedure that would be protective of surface water. In this presentation, the initial study is compared to evaluations using two alternative methods. 7

8 Stanislaus County Study Area Tuolumne 8

9 Framing the Study Acceptable depletion will be established under Groundwater Sustainability Plans (GSPs) by Framing Question for the pre-gsp study: At what distance from the rivers will cumulative streamflow depletion be less than significant for pre-gsp new wells? Assessed impacts from shallow (upper 200 feet) and deep (below 200 feet) at varying distances from rivers. Assuming 10 new wells were to be located close to a groundwater-connected river, at what distance would cumulative depletion be less than 50% of the gaging station error? 9

10 Approach 1 Streamflow depletion was simulated using the USGS STRMDEPL08 analytical code. A conceptual model and aquifer parameters were developed based on the USGS MERSTAN model and data compiled from specific capacity tests. 10

11 Groundwater Extraction Expressed as Streamflow Depletion (percent) Long Term Streamflow Depletion by Wells Completed Above 200 ft near the Stanislaus and Tuolumne Rivers - Low Transmissivity Case T = 2,000 ft 2 /day S y = 0.25 Streambed Conductance (C) = 20 ft/day Immediate response Muted, shifted peak Post pumping peak depletion Pumping 500 ft 2,500 ft 1 mile 2 miles 11

12 Groundwater Extraction Expressed as Streamflow Depletion (percent) Long Term Streamflow Depletion by Wells Completed Above 200 ft near the Stanislaus and Tuolumne Rivers - High Transmissivity Case T = 8,000 ft 2 /day S y = 0.25 Streambed Conductance (C) = 80 ft/day Pumping 500 ft 2,500 ft 1 mile 2 miles 12

13 Groundwater Extraction Expressed as Streamflow Depletion (percent) Long Term Streamflow Depletion by Wells Completed Below 200 ft near the Stanislaus and Tuolumne Rivers - Low T and S Case Aquifer: T = 4,000 ft 2 /day; S = Aquitard: B = 200 ft; S y = 0.25; K = 4 ft/day Pumping 500 ft 2,500 ft 1 mile 2 miles 13

14 Long Term Streamflow Depletion by Wells Completed Below 200 ft near the Stanislaus and Tuolumne Rivers - High Transmissivity Case Groundwater Extraction Expressed as Streamflow Depletion (percent) Aquifer: T = 13,000 ft 2 /day; S = 0.01 Aquitard: B = 200 ft; S y = 0.25; K = 4 ft/day Pumping 500 ft 2,500 ft 1 mile 2 miles 14

15 Approach 2 Streamflow depletion was simulated using AnAqSim, an analytical element modeling code. A conceptual model and aquifer parameters were developed based statistical analysis of 112 specific capacity tests in east Stanislaus County. A stacked three-layer 30 model with a river linesink was 25 developed. Frequency Hydraulic Conductivity Frequency % 80% 60% 40% 20% 0% Cumulative Frequency 15 Estimated Hydraulic Conductivity (feet/day) Frequency Cumulative %

16 Streamflow Capture from Shallow Wells ( ft) Streamflow Capture (percent) Aquifer: T = 1,950 ft 2 /day; S y =0.25 (upper 50 ft above aquifer); S=0.02; K v = 0.13 ft/day; C = 10 ft/day Time (days) Pumping 500 feet 2,500 feet 1 mile 2 miles 16

17 Streamflow Capture from Deeper Wells ( ft) Streamflow Capture (percent) Aquifer: T = 2,200 ft 2 /day; S=0.001; K v = 0.1 ft/day; C = 10 ft 2 /day Time (days) Pumping 500 feet 2,500 feet 1 mile 2 miles 17

18 Cumulative Sensitivity Analysis for Shallow Well 500 ft from River Streamflow Capture (percent) Time (days) Pumping Base Case T=4,350 ft2/day S=0.002 Kv=2.9 ft/day C=100 ft2/day 18

19 Stanislaus County Hydrologic Model Sub-model of C2VSim-FG IWFM finite-element GW/SW model Under development Approach 3 19

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21 Stream Gage 1 Wells Completed Above 200 Feet Groundwater Extraction Expressed as Streamflow Depletion (percent) T ~ 4,500 to 5,000 ft 2 /day S y = 0.16 Streambed K = 3 ft/day Days Pumping 500 ft 2,500 ft 1 mile 2 miles 21

22 Stream Gage 2 Wells Completed Above 200 Feet Groundwater Extraction Expressed as Streamflow Depletion (percent) T ~ 4,500 to 5,000 ft 2 /day S y = 0.16 Streambed K = 3 ft/day Days Pumping 500 ft 2,500 ft 1 mile 2 miles 22

23 Stream Gage 2 Wells Completed Below 200 Feet Groundwater Extraction Expressed as Streamflow Depletion (percent) T ~ 4,500 to 5,000 ft 2 /day S y = 0.12 Streambed K = 3 ft/day Days Pumping 500 ft 2500 ft 1 mile 2 miles 23

24 Study Conclusions Surface Water Protection Zones established based on distance from river: 1 mile for shallow wells 2,500 feet for deeper wells. Applications for wells outside Surface Water Protection Zones do not require further evaluation of surface water depletion. Applications for wells in a Surface Water Protection Zone must include a site-specific Surface-Groundwater Interaction Study. 24

25 Study Conclusions The more sophisticated the model, the more deterministic the practitioner can be. Analytical models are useful, but require a knowledge of local conditions or a range of cases to bracket variability and uncertainty. Distance, horizontal hydraulic conductivity, vertical hydraulic conductivity, and streambed conductance appeared to be the most sensitive factors. Assessment location matters. 25