Well Head Delineation Zone Analysis for Nantucket Island, Massachusetts. Assigned: April 2 nd, 2012 Presentation and Reports Due: April 16 th, 2012

Transcription

1 Well Head Delineation Zone Analysis for Nantucket Island, Massachusetts Assigned: April 2 nd, 2012 Presentation and Reports Due: April 16 th, 2012 Semester Project I NM Tech, H547 1

2 INTRODUCTION The Wannacomet Water Company, the municipal water supplier for the Town of Nantucket, is proposing to add a new well field on the island to meet growing water deman (Pout Pond; Figure 1). Wannacomet Water Company's existing water system primarily includes two groundwater sources: a 12-inch gravel packed well (Wyer s Valley) screened in a confined aquifer; a tubular well field consisting of eighty nine 2½inch wells, approximately (Wyer s Valley) pumping from the water table aquifer; and a 12-inch gravel pack well at the State Forest well field off Lovers Lane Road pumping from the confined aquifer. There is also a shallow, hand dug well at Siasconset (Figure 1). During the past several years, the Wannacomet Water Company has experienced a steady increase in their water demand (Figure 2). Summer pumping demands has almost doubled between 1991 and 1995 due to increases in tourism and year round residents. A pumping test was recently completed by M. Person at a new proposed well field (Pout Pond) and results of this aquifer test are presented below. Pumping represents a small portion of total recharge (Figure 3). The Wannacomet Water Company has hired you (NM Tech Hydrodynamics Group) to determine the groundwater protection zone (ZONE II) for the combined well fields on the island. This will require you to develop and calibrate a MODFLOW of the Island s sole source aquifer and determine the area of contribution of water to these wells through pathline analysis (MODPATH). You must consider pumping both in the shallow water table aquifer (tubular well field at Wyers Valley, hand-dug well at Siasconset) as well as in the deeper confined aquifer (Wyers Valley, State Forest, and the proposed Pout Pond). There are several complications regarding developing and applying a MODFLOW model to Nantucket Island: 1. Representing a series of shallow lakes that modify the water table configuration (see Figure 1). 2. How many layers to include in your hydrostratigraphic framework model (see figures below and PT12 and PT13 test bore lithologic descriptions). 3. Whether to treat hydraulic conductivity as a constant or let it vary in space within each layers. 4. Choosing the appropriate boundary conditions along the edge of the solution domain due to the presence of sea water at depth. The Department of the Environment for the State of Massachusetts will expect you to calibrate your model so that it can reproduce the water table configuration within the shallow, unconfined aquifer as well as historic drawdown data patterns within the semiconfined aquifer in response to pumping matching and average annual recharge. This model calibration exercise will utilize existing USGS monitoring well data (location of wells shown by red circles in Figure 1), drawdown data from Pout Pond and State Forest pumping tests, and the water table contour map (black lines, Figure 1). You will first 2

3 calibrate your model to steady-state conditions using average recharge and pumping conditions. You will then be required to construct a Zone II is determined by computing drawdowns using the calibrated model and setting the pumping rates at the production wells to their capacity for 6 months with no recharge. The maximum pumping rates you should use are listed in Table 1. The resulting cone of depression after 6 months of pumping is used to determine the zone of contribution through reverse particle tracking (MODPATH). Details on how to compute a ZONE II are described in Person et al. (1998). AVAILABLE DATA: You will be provide with the following electronic data sets to conduct your well head delineation analysis: 1. Background information on hydrogeologic properties from prior studies (this report). 2. Well logs with lithologic descriptions (pt12.xls, pt13.xls, mw2004_d5.xls) 3. Hydrologic stress data (recharge and pumping rates; nan_pump_precip_rech_apr3.xls) 4. Water level history as select water table wells (nan_wlobs.txt). 5. Estimates of maximum drawdown in response to pumping around the State Forest and Pout Pond well fields (see Figures 6-13). 6. Water table map (watertable_map_mar31_2012.tif) 7. Information on location of freshwater-saltwater interface (this report). 8. Information on well construction, well elevations, and maximum pumping rates (well_info.xls). NANTUCKET ISLAND HYDROGEOLOGY The principal aquifer on the island package consist of Late Pleistocene glacial outwash sands, gravels, and silts (Figure 1). The outwash sands are disected by a series of leaky, confining units composed of silt and fine sand. Well logs from the Pout Pond, State Forest and Wyers Valley well sites identifies sand and gravel aquifer material to depths of about 200 (pt12.xls, pt13.xls; mw2004_d5.xls; Figure 4). Wisconsian age, glaciolacustrine deposits underly the outwash sands beneath most of Nantucket (Figure 5). These are comprised of thick silt and clays layers found in MW2004-5D between ' depth. These are likely associated with Glacial Lake Nantucket. Similar sediments have been reported on Cape Cod by Masterson, et. al. (1997). For the purpose of this study, you may take the top of the glacio-lacustrine deposits to be the bottom of your model domain. However, the depiction of the hydrogeology of the island is up to you. Sediment logs from the installation of observation wells throughout the Pout Pond, State Forest, and Wyer s Valley well fields show that the sediments are more or les consistent throughout the area (Figure 3). Several aquifer tests were carried out on the island at all three well fields and are summarized in Table 2. Aquifer test results from the Pout Pond 3

4 well field are presented in Figure The location of the Pout Pond wells is shown in Figure 4. Results of a pumping test conducted by Haley and Ward for the State Forest well field are shown in Figures You can use this data to estimate drawdown patterns around these wells. Calculated recharge is about 50% of precipitation. The total groundwater withdraws from State Forest, Wyers Valley, and Siasconset was, on average, 3.5% of computed recharge. Recharge presented in Figure 3 was calculated as the difference between precipitation and evapotranspiration. R = P PET (1) where R is recharge (cm/month), PET - potential evapotranspiration (cm/month), and P is precipitation (cm/month). Potential evapotranspiration (PET) is calculated as a function of average monthly temperatures using a modified Thornthwaite algorithm. (2) (3) where is the monthly average saturated vapor pressure, T is the monthly average air temperature ( o C), and PET is the monthly average potential evapotranspiration. If potential evapotranspiration exceeds precipitation, recharge is set to zero. This approach doesn t take into account, among other things, the physics of unsaturated flow nor soil the effects of moisture deficits on plant transpiration. Individual precipitation events are not well represented using this approach. The approach is most applicable for calculating mean monthly recharge. You should assume that the recharge is representative of actual conditions. A description of available data for your MODFLOW model are presented in Table 3. Precipitation and temperature records were used to estimate recharge (Person et al. 1998). Climate data was obtained from climate stations from the Nantucket Airport and Edgarstown Martha's Vineyard. The groundwater table at the proposed Pout Pond well site location has an average groundwater elevation of about 9.2 feet above sea lev el (Figure 1). The groundwater slopes very gently from the well site toward the ocean to the northwest. Seawater acts as a no-flow boundary to freshwater flow. Electro-magnetic (TDEM) soundings soundings were collected on Nantucket Island at 10 sites in May, 2002, and at an additional 6 sites in August, 2004 by the US Geological Survey in cooperation with Indiana University (Marksammer et al. 2007). TDEM soundings collected along a northsouth transect across Nantucket indicate that the freshwater/saltwater interface may be approximately 360 feet below the surface in the northern and central portions of the island beneath the low permeability glacio-lacustrine clay unit (Figure 5). Saltwater was identified at shallower depths in the southern portion of the island (e.g., fbsl). 4

5 The freshwater/saltwater interface is shallower and extends further landward along the southern portion of the island because the glacio-lacustrine deposit does not extend across the island to the south. To the North, the saltwater appears to be confined to the coastline. Saltwater-freshwater interface acts as a barrier to low salinity, shallow groundwater. PROCEDURE 1. Develop a hydrogeologic framework model for Nantucket Island. 2. Build at steady-state hydrologic model with MODFLOW using average recharge, pumping conditions. Use either the drain option to represent the ponds or treat them as conductive units with a porosity of 1. A constant head boundary at the edge of the island should be imposed for the top layer. It is up to you to decide what boundary condition to impose for lower layers (no flux or constant head). You may wish to refine your model around the well fields. But don t go overboard. In this step, you will determine the most representative values of transmisivity for your aquifers and spatial variations in Transmissivity should you decide to do so. Save this as a separate MODFLOW data deck (*.gpr file/folder). 3. Calibrate this stead-state model using water table contour map, maximum drawdowns reported from pumping tests (Pout Pond and State Forest), and average values of water levels in USGS monitoring wells (Figure 1). You ll need to estimate average water levels from the water level records from these wells (nan_wlobs.txt). 4. Using your calibrated model, apply maximum pumping and no recharge for six months of pumping to compute the head field you will use to construct a well head delineation zone. Save this as a separate MODFLOW data deck (*.gpr file/folder). 5. Using the results from 4, compute a delineation zone for the well fields on the island using reverse particle tracking in MODPATH. Save this as a separate MODFLOW data deck (*.gpr file/folder). In some instances, using a low porosity is needed in MODPATH to get the path lines to reach the water table or no flow boundaries. 6. You will be required to write up your analysis of the delineation zone and make an in-class presentation of your findings on April 16 th. Discuss how well you felt the model is calibrated to your data and possible sources of errors. Original Nantucket hydrogeologic limericks made during your presentation are optional but encouraged (e.g. There once was a hydrogeologist from Nantucket. He measured precipitation with buckets..). 5

6 7. Questions to address in your report are: How well do aquifer parameters from the pumping test compare to best fit values from your model calibration exercise? What hydraulic boundary conditions were needed to match the drawdown patterns in the confined aquifer? Was the model calibration reasonable? Does the delineation zone make sense? (If the delineation zone extends into Nantucket Harbor it doesn t make sense.) REFERENCES Marksamer, Andee J., M. A. Person, F. Day-Lewis, J.W. Lane, D. Cohen, B. Dugan,K. Henk, and M.Willett. Integrating Geophysical, Hydrochemical, and Hydrologic Data to Understand the Freshwater Resources on Nantucket Island, Massachusetts. In Hyndman, D.W., F. D. Day-Lewis, and K. Singha (eds.) Data Integration in Subsurface Hydrology, AGU Water Resources Monograph, 2007, DOI: /172GM12, 17 p. Masterson, J. P., B. D. Stone, D. A., Walters, and J. Savoie, Hydrogeologic Framework of Western Cape Cod, Massachusetts, Hydrologic Investigations Atlas, Ha-741, Person, M. Marksamer, A. J., P. Sauer, K. Brown, D. Bish, L. Litch, B. Dugan, N. Krothe, and M. Willet, 2011, Use of a Vertical δ 18O Profile to Constrain Hydraulic Properties and Recharge Rates Across a Glacio-Lacustrine Unit, Nantucket Island, Massachusetts, USA, Hydrogeology Journal, DOI /s Person, M., Taylor, J. and S. L. Dingman, 1998, Sharp-Interface Models of Salt Water Intrusion and Well Head Delineation on Nantucket Island, Massachusetts, Ground Water, v. 36, p

7 Table 1. Maximum Pumping rate data Max Allowable Pumping Average Pumping Rate (gpm) Production History Well Screened Elevation / Width (ft) Wellfield Rate (gpm) (Month/years) State Forest (PW-13) /02 to 7/ to -128 / 20 Wyers Gravel Pack (PW-12) /91 to 7/ to / /91 to 7/ to 4.5 / 5 Wyers Tubular 1500 Pout Pond (Proposed) 1000 na na -105 to 85 / 20 Siasconset /91 to 7/06 Table 2. Summary of Hydraulic Conductivity and Storativity from Time-Drawdown and Distance Drawdown Analysis Straight line- Analysis Storativity Distance Drawdown Analysis Storativity Well Maximum K(ft/day) Drawdown (ft) State Forest MW E PVC E-05 MW E-03 MW E-04 MW E-04 8" 7.48 Wyers Valley 5a E PW-1 5 Pout Pond E E d E E-04 PT

8 Table 3. File Description for Delineation Zone Project File Name pt12.xls pt13.xls mw2004_d5.xls nan_pump_precip_rech_apr3.xls well_info.xls wt_loc.txt nan_wells_apr19e.txt nan_wlobs.txt nan_conf_wlobs.txt Nanpump_wells.xls nan_pump_delin.txt rech_delin.xys watertable_map_mar31_2012.tif Description Spreadsheet containing drillers well log, Wyers valley rotosonic well (gravel pack well) Spreadsheet containing drillers well log, State Forest rotosonic well Spreadsheet containing drillers well log, Pout Pond rotosonic well Spreadsheet containing time dependent recharge and pumping rates Spreadsheet containing well position and screen intervals GMS formatted text file containing water table position. You need to add layer information to this file. GMS formatted text file containing production and monitoring well locations Steady-state water-levels for unconfined aquifer Steady-state water levels for confined aquifers in the vicinity of Pout Pond and State Forest well fields. Note that static water level at State Forest is 8.8 feet. Static water level at Pout Pond is 9.17 feet. Spreadsheet containing steady-state pumping rates for Wyers valley and State Forest well fields GMS formatted text file containing time dependent pumping rates for delineation zone calculations for Wyers Valley and State Forest Wells gms formatted 'xys file containing time zero recharge rates during delineation calculations image file showing water table location, well positions, island geometry 8

9 Figure1.LocationofUSGSmonitoringwellnetworkwells(bullets)andcontourmap(blacklines) ofwatertableelevations(ft)fornantucketisland,august,1991(fromhorsleyetal.,1991).the tworegistrationpointsarelocatedat(1,740,000,90,000)and(1,740,000,110,000).the maximuminlandlakebedelevations(inft)arelistedinblue. 9

10 Figure 2. Total monthly pumping at Wyers Valley, State Forest well fields on Nantucket Island. 10

11 Figure3.Hydrologicstressesusedinmodelcalibrationexerciseincludingrechargeand pumping. 11

12 Figure 4A. Northeast-Southwest cross-section across Pout Pond well field. Screened depth of wells along this section through the Pout Pond well field is also presented. Aquifer test was conducted in the layer between -85 to -105 feet. 12

13 Figure 4B. North-South cross-section across State Forest well field. Location of monitoring wells is also presented. 13

14 Figure 5. North-South cross-sectional transect showing the position of saltwater beneath Nantucket Island based on TDEM soundings described in Marksamer et al. (2007) in the vicinity of the Wyer s Valley and State Forest well fields. The locations of the geophysical soundings across the island is shown by the square symbols on the map of the island. 14

15 Figure 6. Pout Pond time-drawdown aquifer test analysis using Hantush-Jacob solution for a confined aquifer overlain by a leaky confining unit. The location of this well is presented in Figure 4. 15

16 Figure 7. Pout Pond time-drawdown aquifer test analysis using Hantush-Jacob solution for a confined aquifer overlain by a leaky confining unit. The location of this well is presented in Figure 4. 16

17 Figure 8. Pout Pond time-drawdown aquifer test analysis using Hantush-Jacob solution for a confined aquifer overlain by a leaky confining unit. The location of this well is presented in Figure 4. 17

18 Figure 9. Pout Pond time-drawdown aquifer test analysis using Hantush-Jacob solution for a confined aquifer overlain by a leaky confining unit. The location of this well is presented in Figure 4. 18

19 Figure 10. Distance-drawdown aquifer test analysis using Jacob straight line analysis for State Forest well field. The location of this well is presented in Figure 4. 19

20 Figure 11. Changes in water levels in wells screened in unconfined (2006-3S and ) and deeper confined (2004-1) aquifers during long-term pumping test. The measurements were recorded starting at 10 am on August 2, See Figure 5 for location of well screens. 20

21 Figure 12. Straight-line analysis of long term aquifer test at State Forest Well Field for a monitoring well located 130 feet from production well. 21

22 Figure 13. Straight-line analysis of long term aquifer test at the State Forest well field for a monitoring well located 600 feet from production well. 22