Simulated maximum withdrawals from the Upper Patapsco, Lower Patapsco, and Patuxent aquifer systems in Anne Arundel County, Maryland
2020, Andreasen, D.C.
Administrative Report 20-02-01
Anne Arundel County relies heavily on withdrawals from the Upper Patapsco, Lower Patapsco, and Patuxent aquifer systems for public water supply. In 2018, approximately 33.5 million gallons per day were withdrawn from Anne Arundel County Department of Public Works well fields in the central and northern part of the county. Remaining available drawdown in 2018 in the well fields before water levels reach management levels ranges from 24 to 193 feet in the Upper Patapsco aquifer system, 150 to 500 feet in the Lower Patapsco aquifer system, and 100 to 960 feet in the Patuxent aquifer system.
A previously constructed groundwater-flow model (MODFLOW) was revised, verified for calibration, and used to estimate maximum withdrawals from the Upper Patapsco, Lower Patapsco, and Patuxent aquifer systems at existing Anne Arundel County Department of Public Works well fields (Arnold, Broad Creek, Crofton Meadows and Severndale), and at future well fields (Crownsville and Millersville). Results of the modeling show that a total of 114.4 million gallons per day can be withdrawn from the well fields before water levels reach management levels in one or more of the aquifers. As a result of the simulated leakage between aquifers, the amount that can be withdrawn from deeper aquifers is controlled, in part, by the available drawdown in shallower aquifers. The simulated maximum withdrawal is approximately 1.7 times greater than the 2086 build-out value of 66.5 million gallons per day and approximately 3.4 times greater than the amount pumped in 2018. Simulated water levels are as deep as 155, 295, and 480 feet below sea level in the Upper Patapsco, Lower Patapsco, and Patuxent aquifers, respectively. Drawdown of this magnitude has the potential to cause land subsidence, saltwater intrusion, and well interference with other groundwater users.
The simulated net water budget for the modeled aquifer system for the maximum-withdrawal scenario indicates that recharge applied to the water-table aquifer (100 percent of net inflow to the model) is balanced by a net outflow to rivers (46 percent), wells (31 percent), and constant-head and head dependent boundaries (22 percent and 1 pecent, respectively). As withdrawals increased under the maximum withdrawal scenario, net flow to rivers and constant-head boundaries decreased. The maximum withdrawals result in an approximate 14-percent reduction in net river discharge from the 2018 amount.
Groundwater travel times from Anne Arundel County Department of Public Works well fields pumped in the maximum withdrawal scenario were calculated using the groundwater-flow model and the particletracking code MODPATH. In the Upper Patapsco aquifer system well fields, the minimum travel time from model boundaries (water-table aquifer or brackish tidal surface water) is 30 years for Severndale, 75 years for Arnold, and 277 years for Broad Creek. In the Lower Patapsco aquifer system well fields, the minimum travel times from model boundaries is 135 years for Arnold, 277 years for Broad Creek, 94 years for Crofton Meadows, 144 years for Crownsville, 70 years for Millersville, and 97 years for Severndale. In the Patuxent aquifer system well fields, the minimum travel times from model boundaries is 459 years for Arnold, 375 for Broad Creek, 244 for Crofton Meadows, 234 for Crownsville, and 193 years for Millersville. The well fields with the shortest travel times (less than 100 years) to the water-table aquifer (recharge area) and brackish tidal surface water include the Upper Patapsco well fields at Arnold (75 years to brackish Magothy River) and Severndale (30 years to recharge area), and the Lower Patapsco aquifer wells fields at Crofton Meadows, Millersville, and Severndale (94, 70, and 97 years to recharge area, respectively). Travel times from well fields to model boundaries under the maximum-withdrawal scenario range from approximately 1.4- to 16-times less than under 2018 pumping conditions and 1.1- to 9.9-times less than under 2086 build-out pumping conditions.