Identification of the Area of Influence

An essential element for the WCP plan is the identification of the “area of influence.” The area of influence is the area outside of which the likelihood of Cryptosporidium or fecal

contamination affecting the treatment plant is not significant. Identification of Cryptosporidium sources, associated control measures, and future watershed surveys (see Section 2.2.2.1) will be targeted within this area. Methods to be used to establish the boundaries of the area of influence

are at the discretion of the PWS, as long as the state considers it sufficient to approve the area delineated.

Delineation

Systems may develop their own watershed delineation. The starting point for such a delineation may be the delineation developed by the state as part of the source water assessment program (SWAP). In referencing the delineation prepared in the SWAP process, systems should be careful to understand how the initial delineation was prepared. Different states employed different delineation approaches. To delineate watersheds, some states started with watersheds as catalogued by the U.S. Geological Survey (USGS). The USGS has assigned each watershed and its subwatersheds in the United States a hydrologic unit code (HUC). Because the HUC subwatersheds can be quite large, and a PWS's source may come from only a section of the watershed, or portion of a hydrologic unit, sometimes only the part of the watershed upstream of the PWS's intake was mapped. Sometimes watersheds were further segmented into "critical areas" within which more detailed assessments were

performed. Some states delineated critical areas based on setbacks from the edge of the source water and all tributaries feeding into the source water. Others defined critical areas based on a fixed distance or time-of-travel from the intake (upstream of the intake or in all directions from the intake).

Systems that need to delineate their watersheds or subwatersheds for the first time and do not have GIS available can do so with topographic maps. The first step is to find the source (including tributaries) and the water treatment plant intake on the map. Each of the contour lines (which is actually not a line but a closed shape) around the source connects points of equal elevation. Upstream, the elevation indicated by each contour line increases with distance from the source. All precipitation falling within a zone of increasing elevation around the source will flow towards the source. Where the contour elevations stop increasing and begin decreasing is the break point. On the other side of the break point, water is flowing into a different watershed. The area delineated by connecting the break points is the watershed (AWWA 1999). See

http://www.nh.nrcs.usda.gov/technical/Publications/Topowatershed.pdf for an illustrated fact sheet on delineation. If the intake is not at the downstream end of the watershed, it is only necessary to delineate the area upstream of the intake. Systems with GIS can follow the same process using digital elevation model (DEM) data rather than contour lines.

PWSs using ground water under the direct influence of surface water (GWUDI) as a source can delineate an area of influence by combining a delineation of the watershed influencing the ground water source with a delineation of the wellhead protection area.

Watershed Hydrology

Once the watershed has been delineated, PWSs should examine the hydrology of their watersheds to help determine the area of influence. The analysis submitted to the state must contain information on the watershed's hydrology. Stream discharge can affect the transport of sediment and Cryptosporidium oocysts, especially during and after storms. When more rain falls than can be absorbed immediately by the soil, soil cover, or impervious surface, water will pond on the surface. With increasing rainfall, the water will flow to a lower level on the surface, to a river, lake, or reservoir, as shown in Exhibit 2.3. As water travels, it may pick up contaminants on the soil surface (e.g., Cryptosporidium oocysts from deposited fecal matter). These particles are then suspended in the runoff and can be transported to surface water supplies. The

microorganisms (including parasitic protozoa) associated with the soil can be transported as individual organisms, aggregates of organisms, or within an aggregate of soil particles and organisms.

Exhibit 2.3 Ground Water/Surface Water Interaction

Ground water that is considered to be under the direct influence of surface water is usually immediately adjacent to surface water or to the discharge point of a spring. These ground water supplies are considered vulnerable to contamination by microbial contaminants like

Cryptosporidium (consequently, GWUDI sources are treated like surface water sources under the

SWTR, IESWTR, and LT2ESWTR). GWUDI may be contaminated through direct contamination (e.g., an inadequately protected spring), direct infiltration of oocysts from the surface as a result of rain, and as a result of the action of pumping wells (see Exhibit 2.3). Given sufficiently high pumping rates, wells can locally reverse the direction of ground water flow. In such cases, surface water is induced to flow from a river, lake, or reservoir into the adjacent ground water, where it may be extracted by pumping wells. If the surface water is contaminated with microbial

contaminants, the adjacent ground water may also become contaminated.

Water quality flow models analyze specific hydrologic, geographic, and water quality parameters to estimate the travel time needed for contaminants to reach a drinking water intake and the amount of contamination at that intake. Surface runoff models may also be used to assess the potential impact of individual Cryptosporidium sources, and to identify watershed areas with the greatest potential impact on source water quality. Models should always be validated for the settings in which they are used.

PWSs should also consider topography and soil type, which can affect hydrology. Areas with steep slopes may experience a higher percentage of overland flow or runoff (as opposed to infiltration and subsurface flow) and have faster overland flow rates during rainfall than flat areas.

Cryptosporidium may be more likely to be transported to water bodies in such areas, although if

the topography is very steep, livestock that carry Cryptosporidium may not be present.

can also affect overland flow. Riparian zones can be considered sensitive areas simply due to their proximity to streams that feed into source waters. They are also subject to erosion. PWSs should also factor soil types into their decisions; areas with high clay content may be more impermeable or more subject to erosion and can contribute to high turbidity.