ARCHAEOLOGICAL SITES AT RISK FROM SEA-LEVEL RISE: GROUNDWATER LEVELS AND SALINITY BENEATH JAMESTOWN ISLAND, COLONIAL NATIONAL HISTORICAL PARK, VIRGINIA

Sea-level rise and the resulting changes in groundwater levels and chemistry are predicted to cause significant damage to archaeological artifacts on Jamestown Island, Virginia. To assess this risk for management purposes, long-term groundwater monitoring is being implemented to evaluate Jamestown’s current groundwater hydrology and chemistry, and predict changes.

Forty-six monitoring wells (pipes slotted near the bottom) and piezometers (pipes opened at the bottom) were constructed to depths of 2 to 29 feet around the island. The wells and piezometers were placed in clusters at two individual sites and along three transects from lowland marshes or rivers, to uplands.

Across the island, sediment generally coarsens downward from sandy-silty clay, to fine-grained sand, and then to coarse-grained sand and gravel. At eastern upland sites, specific conductance of groundwater in the fine-grained sand is 7,000 to 15,000 microsiemens per centimeter at 25 degrees Celsius (μs/cm), while specific conductance of groundwater in the deeper, coarse-grained sand and gravel is lower, 2,000 to 6,000 μs/cm. Below eastern marsh sites, specific conductance of groundwater in fine-grained sand is 3,700 to 5,700 μs/cm, while it is 4,600 to 10,000 μs/cm in the coarse-grained sand and gravel. At western upland sites, specific conductance is the lowest, 150 to 3,000 μs/cm. At western sites, the groundwater’s specific conductance increases with increasing proximity to the marsh, to levels comparable to those of eastern marsh sites.

Specific conductance and sediment characteristics indicate that saltwater density and lateral flow from the James River likely control salinity of groundwater beneath the marshes and in the coarse-grained sand and gravel beneath the uplands. Salty groundwater likely introduced by storm surges appears to be trapped in the overlying sandy-silty clay and fine-grained sand beneath the uplands because their low permeability limits the flushing of salty water by freshwater recharge from precipitation. Consequently, lateral intrusions of saltwater and the trapping of saltwater in shallow, fine-grained sediment pose separate risks to artifacts. Continued monitoring and assessment will support improved planning for the protection and recovery of the archaeological artifacts in the face of sea-level rise.