Monday, June 23, 2008 : 3:50 p.m.
Classification Accuracy of Saturated-Thickness Maps for Stratified-Drift Aquifers in New Hampshire
USGS stratified-drift-aquifer maps have served as key references for planners managing New Hampshire water resources for over a decade. This research quantified the classification accuracy of USGS saturated-thickness contour maps. For this purpose, a database of verification wells located in stratified drift was assembled from the driller logs of the New Hampshire Geological Survey. Of the 1300 verification wells, approximately 14% were found to actually be located in till. Saturated thickness was calculated for the 1114 remaining wells, and error matrices of mapped versus actual saturated-thickness classes were constructed. Overall classification accuracies for verification wells in regions that respectively utilized 20 ft and 40 ft saturated-thickness contour-intervals were determined to be 33.7% and 42.5%. Map-user accuracy was found to decline with increased depth. Class-offset analyses revealed that the USGS contoured saturated-thickness data can be described as accurate, but imprecise.
Prior research by the author had suggested that after water quantity and regulatory constraints, only 3.7% (47.6 mi2) of the state’s mapped stratified drift remained available as of 2000 with potential to serve as 150 gpm water supplies (RSDA150). Based on the Mazzaferro yield equation, the results of the current accuracy study imply that one third to one half of the 47.6 mi2 of high-transmissivity RSDA150 in NH may in fact, have insufficient saturated thickness to sustain a 150 gpm well yield. Conversely, between 0.7% (11.1 mi2) and 10.8% (122 mi2) of delineated low-transmissivity regions might have sufficient saturated thickness to sustain a 150 gpm yield. However, setbacks for water quality and aquifer boundaries would lower these last estimates. In addition, such wells would be sparse and difficult to locate.
John A. Lough, M.S., Ph.D., Lakeport GIS/Hydrogeology John A. Lough is the principal of Lakeport Hydrology, and conducts spatial analysis into issues of both water supply and contamination. His expertise includes assessment and projection of stratified-drift-aquifer losses, monitoring superfund remediation sites, investigating the expression of groundwater into surface water; and systemic characterization of bedrock aquifers with fracture-correlated lineament-trace analysis and geophysics. Contact information: (207) 228-6461; jlough1@maine.rr.com; Lakeport Hydrology, 36 Rosemont Ave, Portland, ME 04103.
Russell Congalton, Ph.D., University of New Hampshire Russell Congalton is a professor of remote sensing and GIS in the Department of Natural Resources at the University of New Hampshire (UNH). He conducts basic research involving spatial data uncertainty, accuracy assessment, and validation and applied research in using geospatial information to solve natural resource issues. Contact information: (603) 862-4644; russ.congalton@unh.edu; Dept. of Natural Resources, James Hall, University of New Hampshire, Durham, NH 03824
2008 NGWA Conference on Eastern Regional Ground Water Issues