Mansfield Water Supply Impact Assessment

The New Jersey Coastal Plain consists of several major aquifer zones. These aquifers are separated by leaky aquitards, and it has been documented that diversions in one aquifer can cause impacts to adjacent aquifers as well. Historically, the permitting process for new diversions requires a constant-rate aquifer pumping test with one observation well in each of the aquifers adjacent to the one pumped, in addition to those in the aquifer pumped. The drawdown induced by the test in these adjacent aquifers is measured and reported, but unless the short-term impact is acute, the long-term impacts are not considered further.  During the public comment period for the water allocation permit, residents voiced concerns that the client’s proposed diversion from a deep aquifer might have some impact on residential wells, which were screened in shallower aquifers.  The NJDEP thereupon required the client to determine the expected impacts to wells in the region of the proposed diversion.  They also required a baseline and monitoring system that could be used to gauge impacts in the pumped aquifer and the overlying aquifer after the diversion increase.

In response to a sensitive situation in an area of critically depleted water supply, adjacent to New Jersey’s Water Supply Critical Area No. 2, Hatch Mott MacDonald hydrogeologists undertook to simulate the hydraulics of this multiple aquifer system to better predict the impact of the proposed diversion on individual member aquifers.

HMM created a twelve-layer, three-dimensional, finite-difference model simulating the leaky multiple aquifer system within a 6 mi radius of the proposed diversion. Interactions between the aquifer system and surface water in the local portion of the Delaware River Basin were included in the simulation. The model simulated the flow and storage properties of the aquifer as well as vertical flow and storage in the aquitards. The hydraulics of the pumped aquifer and the adjacent aquitard and aquifer units were calibrated against the drawdown data from the 72-hour constant-rate pumping test, which also allowed the quantification of leakage from adjacent aquifer units through the aquitards. The contributions of more distant aquifer units were simulated based on the published results of other tests. The calibrated model was then used to predict impacts of the now-permitted diversion on the water levels in the Coastal Plain aquifer system, on baseflow to and leakage from surface water in the Delaware River Basin, and on local groundwater flow direction and on the propagation of plumes of dissolved contamination.

Project Highlights:

• GIS was used to compile data from stratigraphic borings and well construction logs into a three-dimensional model of the aquifer system in the region
• The analysis involved compiling the elevation, construction, well performance, and static water level data in order to predict whether the simulated drawdown would be significant for over 1,600 wells and whether the impacts could be addressed by simply lowering a pump setting or if a new well would have to be drilled
• The quality of the well construction data and boring logs varied greatly and the New Jersey Geological Survey commended the considerable effort that we expended to compensate for the poor quality of some of the data that they had provided
• The project required identification of the wells most likely to be impacted and contacting the well owners to invite them to participate in the monitoring study