Hydrologic Assessment of the Shallow Groundwater-Flow System Beneath the Shinnecock Nation Tribal Lands, Suffolk County, New York Active

Background

The Shinnecock Nation is located on the south fork of eastern Long Island adjacent to the town of Southampton to the east, and the hamlet of Shinnecock Hills to the west, in Suffolk County, NY.  Shinnecock Nation tribal lands encompass approximately 1.2 square miles and are bounded to the east and west by tidal creeks, and to the south by Shinnecock Bay. The Shinnecock Nation has a population of 662 residents who live in 256 housing units (U.S. Census, 2016). The Shinnecock Nation became the 565th federally recognized Tribe by the United States government in October 2010. (Shinnecock Nation, 2014).

Shinnecock Nation tribal lands are situated in a coastal environment where bay and wetland health are closely related to changes in amount and quality of the groundwater being discharged to the surrounding water bodies. Oyster fisheries and other marine ecosystems are critical to the livelihood of many residents living on the tribal lands and are vulnerable to contaminated groundwater entering the embayment. Additionally, many residents use domestic-supply wells that tap the surficial aquifer beneath the tribal lands as their primary source of potable water supply. Contamination of the surficial aquifer from flooding (inundation) during intense coastal storms, projected sea-level rise, nutrient loading from fertilizers, and septic effluent have been identified as potential sources of human and ecological health concerns on the tribal lands.

The Shinnecock Nation asked that the USGS help them define baseline hydrologic conditions on the tribal lands to help support decision making by their natural resource team. Characterizing the movement and distribution of shallow groundwater beneath the tribal lands is a critical first step to identifying sources of potential contamination to the surficial aquifer and coastal ecosystems.

Approach

The USGS, in cooperation with the Suffolk County Department of Health Services (SCDHS), facilitated the construction of 17 water-table wells on and around the Shinnecock Nation tribal lands beginning in March 2014. The 17 water-table observation wells are part of a 32 water-table-well network that also consists of existing fire-suppression wells and USGS observation wells. Criteria for selecting the observation well locations included: (1) a representative distribution of the water-table underlying the Tribal Lands; (2) outpost locations that could provide hydrologic boundary conditions and constrain contour lines; and (3) sparsely populated areas, absent of utility infrastructure, that are accessible to the drill rig. Screen zone locations (well points) were determined by offsetting the top of the screen zone approximately 5-ft below the estimated depth to the water table using historic USGS depth-to-water maps (Monti and others, 2013).

Single-base real-time kinematic global navigation satellite systems were used to provide centimeter-level positioning for wellhead surveys. Synoptic groundwater measurements were taken at each well during low and high tides in May and November 2014 to identify changes in water levels due to tidal and temporal (seasonal) influences.  Interpreted contours were created from synoptic water-level measurements to determine the configuration of the water table and to estimate the direction of groundwater flow.  A continuous depth-to-water surface was created by subtracting the interpreted water-table surface from a high-resolution digital-elevation model. The thickness of the unsaturated zone beneath the Tribal Lands will be used to inform plans to rehabilitate or replace existing and (or) failing wastewater infrastructure.

 Six water-table observation wells were instrumented with vented submersible pressure transducers that were programmed to measure and record water level and water temperature at 15-minute intervals. Data from these recorders will be used to quantify the influence of precipitation quantities and tidal fluctuations on water levels.

Daily precipitation data from a National Weather Service meteorological station located in Bridgehampton, NY was used to identify potential precipitation influence on the water table located beneath the Tribal Lands.  Fifteen-minute interval predicted tide-stage data from a National Oceanic and Atmospheric Administation (NOAA) tidal station located at Ponquogue Point, NY was used to identify potential tidal influence on the water table located beneath the tribal lands.  Hydrographs of these wells indicate that water levels are affected by upgradient groundwater flow, precipitation, and changes in tidal stage.

Geophysical surveys and published hydrogeolgic maps of Long Island (Smolensky and others, 1989) were used to delineate the shallow hydrogeolgogy and the freshwater-saltwater interface beneath the tribal lands. These data are crucial for characterizing the flow of shallow groundwater from the recharge zone near Sunrise Highway (highest water levels), to the discharge zone along the immediate coast (lowest water levels). Specific capacity (pump) tests were performed on select wells in the water-table network to characterize the physical properties of the surficial aquifer.  

Benefits

High resolution water-level contour (equipotential lines) and depth-to-water maps will be disseminated to the Shinnecock Nation and other stakeholders as published GIS coverages and datasets. Additionally, a provisional assessment of the shallow groundwater-flow system based on the methodology described above will be published as a USGS report. The results of this research will provide the Shinnecock Nations's Natural Resource Team, cooperating governmental agencies, and other stakeholders with the information needed to assess the vulnerability of the tribal land’s coastal ecosystems and potable-water supply.