Predicting Groundwater Sustainability: What Tools, Models and Data are Available?

Transcription

1 Predicting Groundwater Sustainability: What Tools, Models and Data are Available? Ray Wuolo Barr Engineering Company

2 Assessing Sustainability is All About: Predicting (or describing) how potentiometric heads (e.g. watertable elevations) change in response to pumping Predicting how (not if) changing potentiometric heads affect (and are affected by) surface-water features Deciding if these changes are acceptable (this may be much more difficult that predicting hydrologic effects) Predicting whether permitted groundwater withdrawals will be able to continue into the future Making go-no-go decisions in the face of substantial uncertainty (and working to reduce but not eliminate this uncertainty)

3 Conceptual Model of Hydrologic Processes PRECIPITATION EVAPORATION & TRANSPIRATION SURFACE WATER SYSTEM OVERLAND FLOW STORAGE AND SEEPAGE FROM RESERVOIRS INFILTRATION (UNSATURATED FLOW) WELLS GROUND WATER SYSTEM EXCHANGE WITH SATURATED ZONE SATURATED GROUNDWATER FLOW EXCHANGE WITH SURFACE-WATER BODIES

4 Time-dependent responses are very different SURFACE WATER SYSTEM PRECIPITATION OVERLAND FLOW EVAPORATION & TRANSPIRATION STORAGE AND SEEPAGE FROM RESERVOIRS MINUTES AND HOURS INFILTRATION (UNSATURATED FLOW) WELLS GROUND WATER SYSTEM EXCHANGE WITH SATURATED ZONE SATURATED GROUNDWATER FLOW EXCHANGE WITH SURFACE-WATER BODIES MONTHS AND YEARS

5 Can an equivalent porous media approach be used? (i.e. Is Darcy s Law useful?) Fractured Rock? Porous Media WELLS GROUND WATER SYSTEM EXCHANGE WITH SATURATED ZONE SATURATED GROUNDWATER FLOW q = Ki EXCHANGE WITH SURFACE-WATER BODIES

6 Conceptual Model of Hydrologic Processes PRECIPITATION EVAPORATION & TRANSPIRATION How much of this is permissible? SURFACE WATER SYSTEM OVERLAND FLOW STORAGE AND SEEPAGE FROM RESERVOIRS INFILTRATION (UNSATURATED FLOW) WELLS GROUND WATER SYSTEM EXCHANGE WITH SATURATED ZONE SATURATED GROUNDWATER FLOW EXCHANGE WITH SURFACE-WATER BODIES

7 Groundwater Sustainability Questions: Six Principles of Groundwater Withdrawal

8 Principle 1: Pumping causes the water table to drop ( drawdown ) Well, Mine, Ditch, etc. Q1 h1 drawdown in well The magnitude and extent of drawdown is a function of: Pumping Rate Permeability Aquifer Thickness Recharge

9 Principle 2: More Pumping causes more drawdown Q4>Q3>Q2>Q1 h1 h2 h3 h4 h5 drawdown in well

10 Principle 3: Drawdown will continue to expand until a source of water is found (if a source is available)

11 Principle 4: In most cases, cessation of pumping will eventually result in a return to pre-pumping conditions i.e. Storage is Recoverable

12 Principle 5: Recharge from infiltrating precipitation is BY FAR the largest source for groundwater

13 This applies to confined aquifers, too although indirectly, via leakage Think Aquifer Systems Confining Layer

14 Principle 6: Increased pumping does NOT meaningfully induce more areally distributed recharge

15 Examples of Typical Groundwater Sustainability Issues in Minnesota

16 1. Sustainability among competing users of groundwater (i.e. Well Interference ) LOW PERMEABILITY LAYER (AQUITARD) NON-PUMPING CONDITIONS

17 1. Sustainability among competing users of groundwater (i.e. Well Interference ) LOW PERMEABILITY LAYER (AQUITARD) ONE WELL PUMPING

18 The water withdrawal could be a dewatered mine LOW PERMEABILITY LAYER (AQUITARD)

19 1. Sustainability among competing users of groundwater (i.e. Well Interference ) Reduce well capacity via reduced available drawdown LOW PERMEABILITY LAYER (AQUITARD) THREE WELLS PUMPING

20 2. Preserving Available Drawdown for Future Users LOW PERMEABILITY LAYER (AQUITARD)

21 3. Aquifer Mining (continued removal of stored water) Pumping Exceeds Recharge

22 4. Inducing drawdown below a confining layer LOW PERMEABILITY LAYER (AQUITARD)

23 5. Sustainability of pumping with respect to groundwater-dependent natural resources Streams (especially trout streams) Wetlands (especially calcareous fens and riparian wetlands) Recreational water bodies Mine pit lakes (recreation and water supply)

24 Sustainability Issues Vary with Hydrogeologic Setting in Minnesota Paleozoic rocks represent a much larger storage reservoir than geologic units in other parts of Minnesota Hollandale Embayment Karst Areas From Mossler (2008)

25 Hydrologic Data Availability is not Uniformly Distributed??? Localized Data Availability Iron Range Hydrologic Data Aquifer Tests & Detailed Hydrologic Characterization Data availability determined by population and past groundwater use Characterized Geology Hollandale Embayment Karst Areas Dye Tracer Studies From Mossler (2008)

26 The Most Important Prerequisite for Evaluating Sustainability: The Water Balance

27 No-Flow Boundary Groundwater flow into a gaining stream is equal to the recharge minus pumping in the basin* PRECIPITATION EVAPOTRANSPIRATION LAKE-WETLAND SEEPAGE RUNOFF GROUNDWATER FLOW Low-Permeability Base * Assuming Steady-State Conditions (i.e. storage does not change)

28 Stream Baseflow gain across a groundwater system is an indicator of Basin Yield Q2 baseflow Q1 baseflow = Recharge Consumptive Use Q1 baseflow Q2 baseflow

29 STREAM-FLOW MEASUREMENTS ARE INHERENTLY IMPRECISE (that doesn t mean they are not useful) MEASUREMENT ERRORS SCATTER FLOW

30 Basin Yield for the Metro Area has been difficult to measure St. Croix River Recharge estimate of about 1,100 Million Gallons per Day (MGD) or about 5.2 inches per year Norvitch et al., 1973 Recent measurement efforts have results for baseflows in the range of measurement error

31 Baseflow in streams Can be an estimate of groundwater flux for a simple aquifer system Can be a means for estimating aereally averaged recharge Are an important puzzle piece but NOT the whole story

32 Aquifer ( Pumping ) Tests as an indicator of sustainability LOW PERMEABILITY LAYER (AQUITARD)

33 Direct responses to pumping Estimates of K and S Leaky Confined Aquifer Theis curve Unconfined aquifer Approximations of leakage characteristics of confining units Boundary effects/locations

34 Pumping Tests: the good Obtains aquifer parameters at a scale similar to proposed withdrawals Averages out smaller-scale inhomogeneities May show indirect effects of leakage and boundaries Tremendous data set for some groundwater model applications

35 Pumping Tests: the not as good Many tests cost a lot (upfront investment with potential risk of failure) Non-unique analytical solutions Difficult to measure effects on surfacewater bodies Data noise (precip, seasonality, etc.)

36 Water-balance components and controls are typically complex Spatially distributed recharge Focused Recharge (lakes, wetlands) Pumping in Shallow Aquifers Regional Discharge Zones (Rivers) Head-Dependent Leakage Across Aquitards Pumping in Deep Aquifers Regional Flow

37 Numerical Groundwater-Flow Models for Evaluating Sustainability

38 Why a Groundwater-Flow Model? It implicitly encompasses the water balance for an area (from a saturated flow perspective) Area-specific when calibrated Useful makes predictions of future conditions Predictive uncertainty can be quantified (to a point)

39 Common criticisms of models Models are non-unique Predictions have uncertainty Uncertainty (when quantified) proves to be much higher than anticipated (or admitted to?) Models are too complex Models are too simple Models have assumptions Models are biased Data contradict model results/parameters

40 Overcoming Model Phobia Models are used all the time (we just don t recognize them as models) Models are useful in overcoming some (not all) limiting assumptions Think of models as descriptions of the water balance Models can help us focus on what is most important for the problem at hand

41 Many codes can do the job Distributed Parameter codes MODFLOW (finite difference); FEFLOW (finite element) Analytic Element Method codes GFLOW; MLAEM Superposition of simpler analytic solutions (e.g., image wells)

42 What do these models need? Quantification of sources of water (recharge) and distribution Mechanisms for removing water (rivers, wells) Aquifer/Aquitard Geometries Distribution of Hydraulic Conductivity Storage parameters (?) Calibration Targets

43 Sources of Water: Recharge Estimates and Data Availability infiltration rate T resulting piezometric surface

44 Stream Baseflow gain across a groundwater system is an indicator of Recharge Q2 baseflow Q1 baseflow = Recharge Consumptive Use Q1 baseflow Q2 baseflow

45 Estimating Recharge from Stream Baseflows January-February Flows January and February Flow Data May be Reflective of Groundwater Baseflow

46 There are other Stream-Hydrograph methods for estimating recharge Streamflow Recession Displacement (Rorabaugh) Watershed Characteristics Method (Shmagin and Kanivetsky)

47 Examples Surface-Process Models for Calculating Recharge XP-SWIMM; HSPF MIKE-SHE SWB

48 (Adapted from Dripps and Bradbury, 2007) Surface Water Balance (SWB) Model for Recharge Daily Precipitation Temp, Julian Day Wind EVAPOTRANSPIRATION Topography (30-m Digital Elevation) Land Use (including imperviousness) Soil Type (Hydrologic Classification) SURFACE RUNOFF RECHARGE

49 SWB Recharge Output Mean recharge m grid Legend Recharge (in/yr) Water Body I Miles

50 Inverse modeling can be applied to back-calculate recharge

51 What role does suburbanization have on recharge rates? Does moving from agricultural land to lowdensity residential decrease ET? Does routing of storm-water runoff to infiltration basins result in more overall recharge? An increase in recharge of 1 inch over one township results in 625 million gallons of water into an aquifer system

52 Pumping Well Data

53 Calibration Targets Head (groundwater elevation) data CWI is a primary source Errors in location, different times, etc. Virtue of high number of observation Base-flow data extremely important but uneven accuracy and distribution Drawdown observations from pumping tests Historical water levels

54 Optimization methods are standard calibration tools PEST, UCODE Simultaneous calibration to thousands of observations Permits quantitative estimate of predictive uncertainty in some cases Provides an indication of relative sensitivity of parameters to the calibration No guarantee that a more unique solution will be obtained

55 Drawdown (m) Time (days) Combinations of steadystate and transient data (e.g., pumping tests) can be used

56 Data needs (generalized) Interpretations of aquifer/aquitard extent and geometries (hydrogeologically focused atlases) Aquifer test data (not just parameters) Base-flow data Pit water elevation/pumping histories (Iron Range)

57 Closing thoughts on tools and data What are regional storage parameters for the Metro Area i.e. can we tell if we are depleting storage? Are we at a steady state in the Metro Area or are water levels dropping? How do lakes contribute to overall recharge (particularly in the Metro Area)? How do we address conduit flow (karst) on a regional scale? When can we not assume EQM?

58 Closing thoughts on tools and data How do we use models (and other analysis tools) to direct data collection? i.e. how can we use models to be proactive rather than reactive? How do we grow models, rather than reinventing the wheel? How can we use models without overselling them How do we communicate and live with predictive uncertainty?