USE OF AN INFILTRATION TRENCH TO OFFSET MINE DEWATERING EFFECTS

The N.C. Division of Water Resources investigated the use of an infiltration trench to offset the effects of mine dewatering from April 2005-June 2006. Dewatering associated with limestone mining in Onslow County, N.C. had resulted, locally, in dramatic lowering of water levels within the shallow aquifers, as documented in nearby residential wells, and on nearby spring-fed lakes. Limited subsidence was also noted.

The site is located within the central N.C. coastal plain, and consists of unconsolidated Cretaceous sediments overlain by 250 ft of Eocene Castle Hayne Limestone, and capped by ~20 ft of Pleistocene sands and clays. Surficial sands combine with the upper limestone beds (a shell-moldic biosparudite) to form the unconfined aquifer. The deeper limestone strata and the underlying Cretaceous Peedee Aquifer are considered locally-confined.

In Spring 2005, a “T-shaped” infiltration trench was excavated between the mine pit and the recreational lakes. Fifteen boreholes were drilled into the limestone and charges set off in order to increase fracturing, and improve secondary porosity. Discharge water was redirected to the trench in April 2005. Effects were monitored through a network of 54 wells screened within the unconfined and deeper aquifers.

Effects of the trench within the shallow Pleistocene wells could not be discerned from rainfall events, excepting one well located adjacent the trench. Responses were observed in several deep Castle Hayne Fm wells along preferential pathways (faulting?), but water levels in the majority of the deep wells mirrored a regional groundwater decline. In contrast, all wells at lateral- and down-gradients to a distance of one mile from the trench, and screened within the upper Castle Hayne Fm exhibited clear and distinct responses to the trench. The magnitude and lag time of the responses generally varied with distance along direction of flow. Within eight months, effects could no longer be discerned even in the closest wells, possibly due to siltation within the trench itself.