Project Overview

Full Title
Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies - 2009 Updates

Location
 Statewide

Cooperating Agencies
Public Water Supply Section
Source Water Assessment and Protection Program

Project Chief
Silvia Terziotti
seterzio@usgs.gov

Period of Project
1/2009 - 6/2009

Team Members
Kirsten Tighe
Ramona Traynor

Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies - 2009 Updates

Project Summary

Problem

The 1996 Amendments to the Safe Drinking Water Act require that each state prepare a source-water assessment for all public water supplies. States are required to (1) delineate source areas supplying wells and surface-water intakes, (2) inventory potential contaminant sources within the delineated source areas, and (3) determine the susceptibility of wells or intakes to the inventoried potential contaminant sources.

In North Carolina, the lead agency charged with this task is the Public Water Supply Section (PWSS) of the Division of Environmental Health, Department of Environment and Natural Resources (DENR). North Carolina's Source Water Assessment Program (SWAP) provides assessments of each public drinking water intake in North Carolina. These assessments provide a relative susceptibility rating calculated using state-wide data.

The U.S. Geological Survey (USGS) cooperated with the PWSS to provide components of the inherent vulnerability rating that was used with the contaminant rating to determine the overall susceptibility rating of source water supplies. To assist the PWSS with rating ground water inherent vulnerability, the USGS compiled the unsaturated zone rating based on the following characteristics: vertical hydraulic conductance, land surface slope, land use and land cover. The watershed characteristic rating includes average annual precipitation, land surface slope, land use, land cover and groundwater contribution.

The SWAP program has planned for updates to the initial ratings as conditions change and new data sources become available. The initial data used to calculate the unsaturated zone ratings and watershed characteristics ratings were compiled from sources from the late 1990's. Critical data layers have been updated since the ratings were calculated. These improved data layers need to be used in the ongoing update of the susceptibility ratings.

Objectives

The objectives of the 2009 update project were to:

1. Update key data layers used for unsaturated zone rating.
2. Update key data layers used for watershed characteristic rating.

Approach

The PWSS determined that selected data layers for the ratings were in need of update because either the resolution of the base data had improved over the earlier analysis or because conditions had changed substantially since the earlier analysis. The following table summarizes the data layers that were updated in this project:

Table 1. -- Layers targeted for update.

Layer used in Ratings	Update applies to Watershed (WS) or Unsaturated zone rating (UZR) characteristics	Approach to update layer
Annual Average Precipitation	WS	Update from the Parameter-elevation Regressions on Independent Slopes Model (PRISM) 1961-1990 to the PRISM dataset 1971-2007 : download, 1971-2000 and individual years 2001-2007, combine layers, average, project, resample, classify
Land-surface Slope	UZR and WS	Update from the 30m National Elevation Dataset (NED) downloaded circa 1998 to the NED downloaded 2008 which contains LiDAR-derived elevation updates from 2001-2005: download, project, resample, classify for UZR and WS separately
Land cover / Land use	UZR and WS	Update from 1992 National Land Cover Dataset (NLCD) to the 2001 NLCD: project, determine factor values for new land cover classes, classify for UZR and WS separately
Vertical hydraulic conductance	UZR	Combine updated layers to create classes from Soils, Kv and depth to water (not updated)
- Soils	UZR	Update from 1:250,000-scale State Soil Geographic database (STATSGO) to available 1:24,000-scale Soil Survey Geographic database (SSURGO) data for 10 counties in North Carolina: download SSURGO soils, calculate depth to bedrock, soil thickness, and permeability, project, classify
- Kv saprolite value	UZR	Update using new values for Piedmont and Mountains that have been acquired from recent groundwater studies in this region of the State.
Interim Watershed characteristics rating	WS	Update ratings using new layers, classify
Unsaturated zone rating	UZR	Update ratings using new layers, classify

These data layers were compiled using the same methods developed for "Rating unsaturated zone and watershed characteristics of public water supplies in North Carolina, a pilot study" (Eimers and others, 2000). The project was jointly funded by the PWSS and the USGS during 1999. A report was published that outlines the methods used for the project (Eimers and others, 2000). Methods for developing the Statewide layers for the unsaturated zone characteristics rating and the watershed characteristics rating are described in online Open File Reports (Terziotti and others, 2001 and Eimers and others, 2001). Scripts are included in the metadata for processing each of the factors. These scripts only needed to be modified slightly to update the methodology used in the original project.