COI Funded Project: Submarine Groundwater Discharge: Identification and Quantification via Remote Sensing, Hydrologic Sampling, and Geochemical Tracers

Ann Mulligan, Marine Policy Center and Matthew Charette, Marine Chemistry and Geochemistry

Project Funded: 2002

Proposed Research

Groundwater discharge to coastal waters can have a significant impact on local ecological structure because of its low salinity and, often times, high nutrient content. Proper management of coastal water resources requires that the quantity and quality of significant inputs be adequately characterized. Groundwater input to coastal waters, often termed submarine groundwater discharge (SGD), is particularly difficult to characterize because it can vary significantly through space and time. While hydrologic measurements can quantify fluid flux at specific points, the spatial variability of flow can lead to significant errors when making estimates using point data. Geochemical tracers, on the other hand, are useful in quantifying total groundwater flux to a surface water body, but are unable to identify the spatial distribution of flow.

We propose to examine the utility of remote sensing technology, specifically airborne thermal imaging, in identifying key groundwater discharge locations to coastal surface water bodies. Hydrologic measurements will be collected before and after the imaging, with the sampling locations after the remote sensing work chosen based on image results. We will also collect surface water and groundwater samples for radium and dissolved nitrogen analyses. The radium samples will be used to estimate groundwater discharge to the water bodies and will be compared with the hydrologic estimates. Nitrogen samples will be used, together with the flow estimates, to determine nitrogen loading to the coastal waters. We propose to conduct the remote sensing at three locations in southeastern New England: Quonochontaug Pond in Rhode Island and Waquoit Bay and off the west coast of Truro on Cape Cod. Previous work in these three locations indicates that groundwater fluxes in these three areas vary by one to two orders of magnitude.

Project Summary

In September, 2002, an aerial thermographic survey of Ninigret Pond, (southern Rhode Island) and Waquoit Bay, (Cape Cod, Massachusetts) was conducted. These images depict the surface temperature at the time of the survey in (a) northwestern corner, (b) Waquoit Bay; and (c) Ninigret Pond. Dark grey indicates cooler temperatures and white represents warmer temperature. The dark grey water adjacent to the coastline shows where cool groundwater is discharging into the warmer surface water. In Waquoit Bay, groundwater and surface water temperatures at the time of the survey were measured in-situ as ~13° C and ~20-21° C, respectively.

Groundwater, flowing from the upland watershed to coastal embayment can be a significant source of freshwater to coastal waters. This groundwater carries retained chemicals and can be one source of excess nutrient loadings in some ponds. Thus, groundwater may provide a previously unanticipated load directly to the coast.

The shallow sediment (e.g., shallowest 10m) in Waquoit Bay consists of medium to coarse sand and is generally regarded as having relatively homogeneous properties. Based on this geology, we hypothesized that groundwater discharge would be relatively uniform along the coast. However, the Waquoit Bay image clearly shows that groundwater discharge is not uniform. Instead, heterogeneities in the subsurface are likely present and are responsible for the non-uniform discharge pattern. Similarly, discharge to Ninigret Pond appears as point sources with relatively low to no discharge occurring along most of the coast. This non-uniform discharge will have a significant effect on estimates of nutrient loadings to these ponds. In particular, the images show that groundwater flux and nutrient loading estimates based on extrapolating point measurements obtained without knowledge of the discharge pattern may result in large errors. Finally, these images indicate that additional work is needed to understand the effect of small-scale geologic heterogeneity on groundwater discharge patterns.