Hydrogeology and ground-water quality at a land reclamation site, Neshaminy State Park, Pennsylvania

At Neshaminy State park, the most important aquifer is the informally named 'Trenton gravel' of Pleistocene age, which consists of poorly sorted sand and gravel. This is underlain by less permeable crystalline rock that limits the downward movement of water. Up to 5 feet of Holocene (or perhaps Pleistocene) alluvium consisting of clay and silt was deposited above the Trenton gravel, but much of the surficial material is dredge spoil, mostly sand and silt from the Delaware River.

Normally, ground water discharges to the Delaware River and Neshaminy Creek. However, because the Trenton gravel is confined by the alluvium and is hydraulically connected to the Delaware River throughout its entire thickness due to channel dredging, hydraulic-head variations in the river are readily transmitted to wells. Thus, flow is toward the Delaware River and Neshaminy Creek at low tide, but the river and and creek recharge the aquifer as the tide rises.

In September 1981 and October 1983, infertile dredge spoil from the Delaware River at Neshaminy State Park was mixed with composted sewage sludge and seeded to grasses to produce a soil product that has not adversely affected ground-water quality as of March 1985. Analyses for pH, specific conductance, nutrients, and heavy metals in ground water at the beginning of the study indicate that the chemistry of the ground water in the dredge spoil at the site differs from that in the Trenton gravel.

Analyses of ground-water samples collected after the first two sludge applications (120 tons per acre and 450 tons per acre), indicate that no significant change occurred in the chemistry of the samples from the Trenton gravel, whereas organic nitrogen increased temporarily in ground water from the dredge spoil 6 months after the larger of the two sludge applications, but quickly returned to background levels. The lack of chemical change with time in the ground water implies either that little of the more than 100 inches of precipitation that fell from April 1983 through March 1985 reached the water table or, more likely, that a mechanism exists beneath the soil- factory site that retards or prevents the downard migration of contaminants.