Elizabeth M. Strange, Diana R. Lane, and Charles N. Herrick

OBJECTIVES:

The purpose of this project was to provide utilities with the information and tools they need to plan for and respond to large-scale vegetation change in their watersheds. Specific objectives were to (1) help utilities better assess risks of large-scale vegetation change, (2) provide information on management strategies to reduce risk or mitigate impacts, and (3) provide a series of case studies to document utility experiences and "lessons learned."

BACKGROUND:

Watershed vegetation plays an important role in protecting drinking water quality and quantity. However, even the best watershed protection programs may fail to plan for potential large-scale alterations to vegetative cover that can threaten drinking water supplies. Utility managers need guidance on actions that can be taken to minimize the risk of harm from these events and to avoid severe impacts to water treatment processes.

HIGHLIGHTS:

Three main findings from the research are that (1) effects of large-scale changes in watershed vegetation involve a few key watershed processes and water quality parameters; (2) the need for changes in treatment processes depends on whether changes in key physical, chemical, and microbiological water quality parameters are outside the range for which water treatment facilities were designed; and (3) both prevention and response strategies are available to water utilities to reduce the risk of impairment to drinking water supplies and the need for costly treatment changes.

APPROACH:

Information for this report was gathered from a structured literature search of published and "gray" literature on the effects of catastrophic changes in watershed vegetative cover on water quantity and quality and from a series of case study interviews with individual utilities. The focus of the literature search and utility interviews was to identify information of specific relevance to drinking water treatment.

RESULTS/FINDINGS:

Results highlighted many commonalities between large-scale events with respect to their impacts on vegetation and soils and subsequent impacts on water quality and quantity. For all events, losses in vegetation can lead to increases in water volume by (1) increasing surface runoff and (2) decreasing water uptake by vegetation. Intact, well-functioning terrestrial ecosystems also are critical to maintaining water quality through soil stabilization and control of nutrients and other pollutants. For water utility managers, the key issue is whether a particular event will pose a risk of producing variability in key water quality parameters outside the range for which their water treatment facilities were designed. In these situations, utilities can attempt to meet "the influent challenge" with significantly modified operating practices or redesign of facilities, or choose to engage in watershed management actions that can prevent or reduce the risk of impacts from large-scale events. Some events, such as catastrophic wildfire and insect infestations, can be prevented or minimized by watershed management. When events occur, management strategies are available to minimize the harm to source water.

IMPACT:

Although catastrophic events may have a low probability of occurring, the potential impacts can be substantial, and therefore these events should be considered as part of any source water protection program. These research results provide information to help utilities engage in planning efforts that identify the risks of particular large-scale events, identify key vulnerabilities and treatment concerns for each type of event, identify specific actions that may be needed before, during, and after events, and create a toolbox of mitigation techniques specific to a utility.

RESEARCH PARTNER:

U.S. Environmental Protection Agency

PARTICIPANTS:

The project was supported by contributions of six partner utilities: City of Calgary Water Resources, Greenville Water System, Orange Water & Sewer Authority, Philadelphia Water Department, Salt Lake City Public Utilities, and the San Francisco Public Utilities Commission.