U.S. Geological Survey
Reston, Virginia
Extended Abstract: Source Water Protection 98, A Technical conference sponsored by the National Water Research Institute, U.S. Environmental Protection Agency, and U.S. Geological Survey, Dallas, Texas, April, 1998.
Last year the U.S. Environment Protection Agency (EPA) published the report, "State Source Water Assessment and Protection Programs Guidance" [3] to meet requirements of the Safe Drinking Water Act Amendments of 1996 (P.L.104-182). The document describes elements of an EPA approvable state Source Water Assessment Program (SWAP) and recommendations for a state Source Water Protection (SWP) Program. The guidance is necessarily general and broad in scope to account for the range of possible situations that individual states may face when developing SWP's. A key element of an SWP is the delineation of source-water areas and associated determinations of the susceptibility of a public water system (PWS) to contamination. Factors that require consideration include identification of potential contaminant sources and determination of the significance of those sources to the PWS in terms of spatial location and transport of the contaminant to the intake of the PWS. The EPA suggests "segmenting" watersheds (fig. 1) for such assessments to rank the significance of sources of contamination within a source area. There is no specific guidance provided by EPA for the segmenting process, but the objective is to "allow the state to focus the actual source-inventory efforts on those types of contamination sources that are considered to be significant in each segment." Accordingly, there are many plausible approaches that could be used to segment the watershed. Hydrologic factors, such as the travel times and delivery ratios associated with the stream network, are among the most obvious. Therefore, a need exists for states to have a consistent methodology and the necessary tools to attempt this type of susceptibility approach. Finally, the problem of segmenting source areas is especially daunting for large interstate source areas where the task of compiling and analyzing information on contaminant sources and hydrologic conditions is more difficult.
Using data from about 400 monitoring stations, the U.S. Geological Survey has recently developed a national water-quality model (SPARROW; [2]) to assess watershed contamination for the conterminous United States. This model and it's digital framework (derived from EPA's river reach file RF-1; [1]) provides a nationally consistent method for segmenting large source areas. Although additional calibrations are required to account for targeted contaminants, the USGS model can be used to address a variety of questions, including travel times, probabilities of exceedance of concentrations for contaminants at selected locations within the source-area watersheds, and relative contributions of different sources and subbasins to contaminant concentrations near the PWS intakes. The USGS plans to work with the EPA to revise the model to suit specific needs of the SWP.
Time-of-travel segmentation
The average stream density of the river network in SPARROW for the conterminous United States is about 0.07 km of channel length per square kilometer of watershed area. The RF-1 network consists of about 65,000 stream reaches that represent about one million km of total channel length. As such, the model is appropriate for evaluation of source areas larger than about 10,000 km2 (or about 50 reaches). For smaller source areas, one can apply the SPARROW method to a more detailed stream network, such as EPA's RF-3, which has more than 600,000 reaches within the conterminous U.S. RF-1 includes average stream velocities for each reach, based on regression equations that relate velocity to long-term mean streamflow and stream order. The regressions were calibrated by using USGS time-of-travel studies. The reaches associated with 2,100 large reservoirs (normal capacity greater than 6,000,000 m3) were also designated in the reach attribute file.
An example application of the RF-1-based model for the Susquehanna River above Havre de Grace, Maryland is presented in figure 2 [see also figure 2 in color]. The travel times to the outlet of the basin were determined for each reach in the stream network. From this analysis, the watershed is segmented on the basis of travel times to a hypothetical water-supply intake at Havre de Grace, MD. This technique could also have been applied at any other point in the basin, including locations of other PWS intakes. Travel-time analysis would provide segments of about equal mean travel time, which could be combined with contaminant-source information to help managers make determinations on the relative hazards posed by contaminant sources in different parts of the basin. This type of analysis is also useful for spill response planning.
Probability of exceedence of targeted contaminants at selected locations in the source area
SPARROW was originally developed as a means to provide regional interpretations of water-quality monitoring data [2]. The model relates measured transport rates in streams to spatially referenced descriptors of contamination sources and land-surface and stream-channel characteristics. Regression equations were developed for long-term mean annual transport of total phosphorus and total nitrogen [2] and could be developed for other contaminants where data are available. By using these equations, probability distributions for the exceedence of specified concentrations of targeted contaminants could be generated for selected locations in the watershed. Note that probabilities here refer to annual mean concentrations; probabilities for short-term exceedence may differ greatly.
Delivered yield of contaminants
Another application of SPARROW models is to characterize individual subbasins within a source area in terms of their contribution to the total mass of contaminant material that is delivered to drinking water intakes (or other stream locations of interest). Together with time-of-travel segmentations, such estimates can be used to prioritize risk-reduction efforts in the source area. In figure 3 [see also figure 3 in color], the contributing areas to each reach in the Susquehanna Basin are shaded to indicate the "delivered yield" of total nitrogen to the location of Havre de Grace, MD. Delivered-yield values integrate the effects of source magnitude with the effects of the loss processes that operate on contaminant material as it travels downstream through the watershed.
The US Environmental Protection Agency (EPA) source-water protection program requires some technically demanding watershed analyses. The US Geological Survey is planning to work cooperatively with the EPA to enhance an existing water-quality model (SPARROW) to address some of these needs. The current and planned capabilities of SPARROW include estimates of travel times useful for segmenting watersheds, probabilities of exceeding concentration thresholds for targeted contaminants near public water supply intakes (PWS), and estimates of the "delivered" yield of targeted contaminants reaching PWS intakes. The benefits of the model include its physically based approach, its ability to incorporate multiple contaminants by using an empirical methodology, and a nationally consistent method that is based on EPA's existing digital stream networks (RF-1). The first step in making the model useful for the source-water protection program would be the addition of PWS intake locations to the RF-1 network. This step would make the model operational for segmenting large (greater than 10,000 km2) source areas that are based on time of travel. Future versions of the model could include targeted contaminants, high and low streamflow analyses, and an RF-3-based spatial scale so that smaller source areas could be analyzed.
1 DeWald, T.R., R. Horn, R. Greenspun, P. Taylor, L. Manning, and A. Montalbano. STORET Reach Retrieval Documentation, 1985. U.S. Environmental Protection Agency, Washington, D.C.
2 Smith, R.A., G.E. Schwarz, and R.B. Alexander, Regional interpretation of water-quality monitoring data. Water Resources Research, 1997. Vol. 33, No. 12, p 2781-2798.
3 U.S. Environmental Protection Agency. State Source Water Assessment and Protection Programs Guidance, Final Guidance, 1997. EPA 816-R-97-009.
