THE IMPACT OF DITCH-PLUGS ON THE HYDROGEOLOGY OF MARGINAL POOLS IN THE SPRAGUE RIVER MARSH, PHIPPSBURG, MAINE

The “grid ditching” of salt marshes is a 400+ year old practice of marsh management that results in an effective draining of surface water on the marsh and loss of pool habitat. Recent restoration projects designed to remediate the effects of grid-ditching (i.e., installation of ditch-plugs) on the Sprague River Marsh, in Phippsburg, Maine, were initiated by the USFW in 2002. Little is known about the effect of these ditch-plugs on the hydrogeology or vegetation of the marsh. The focus of this study was to monitor groundwater levels, movement, salinity, pH and dissolved oxygen through several tidal cycles in two areas located along the margins of the marsh: one of the areas had a ditch-plug installed in 2002 (Southern Alcove, or SA), the other did not (Northern Alcove, or NA).

In the NA, groundwater flows from the margin to the center of the marsh. Groundwater chemistry is heavily influenced by fresh surface and groundwater inflows derived from the uplands. The vegetation grades from high to low marsh towards the center of the marsh. Groundwater levels in the SA are relatively high, and the ground is almost always saturated, suggesting poor drainage in the area behind the ditch-plug. The groundwater in the SA flows along a trajectory that is parallel to the boundaries of the marsh (from north to south), rather than towards the center of the marsh. The vegetation in the SA is dominated by low marsh vegetation. Our evidence suggests that ditch-plug restoration has resulted in adverse effects on the hydrogeology and vegetation of the marsh, and that additional studies of the restoration method are warranted.