ASSESSING ESTUARINE GROUNDWATER NUTRIENT LOADING BY THERMAL IMAGERY AND FIELD TECHNIQUES VERIFIED BY PIEZOMETRIC MAPPING: A METHODOLOGY EVALUATION

Contamination of coastal waters from groundwater discharge is a considerable problem that historically is difficult to assess. A comparison of methodologies for assessing nutrient loading from groundwater showed thermal imagery and field techniques can be a powerful and affordable tool for evaluation of nutrient loading. This methodology was compared with the more common approach to predict groundwater flow rates using piezometric mapping and aquifer characterization.

In April of 2000 a series of thermal infrared aerial surveys were flown over The Great Bay Estuary in coastal New Hampshire. Groundwater discharge was delineated in a large-scale ecosystem, by covering nearly 50 miles of shoreline. One survey was completed in an afternoon and the images were available shortly thereafter, with no post-processing. A subset of suspected groundwater discharge zones were identified, characterized for hydrologic parameters, and sampled for water quality. Flow estimates from thermal imagery and field measurements were then compared with those predicted from piezometric mapping (comprised of nearly 300 monitoring wells) and aquifer parameters.

The two methods have shown remarkable agreement. Thermal infrared has the advantage that it can be used to identify exact locations of groundwater discharge, which in some cases behave as point sources. Other flow assessment methods assume uniform diffuse discharge. However areas with a diverse stratigraphy and/or bedrock influence can exhibit a combination of concentrated and diffuse discharge zones. This is true in inland or shallow estuaries where accumulation of marine clays occurs. The accuracy of estimates from piezometric mapping suffers with complex subsurface conditions or limited site characterizations. In these locations thermal imagery can be especially useful as a direct assessment of groundwater discharge, and may provide more reliable estimates. With the thermal imagery, groundwater discharge is evaluated directly, without the need to evaluate or address upgradient factors. Where zones of high nutrient loading are identified, a detailed characterization of upgradient conditions may ensue.