Abstract
Communities are routinely faced with making watershed land use and wastewater management decisions with limited data. Local governments need tools that make the most of available information to target high risk locations for pollution movement and to evaluate current and future impacts of on-site systems. The purpose of this presentation is to describe an assessment approach communities can adapt to direct local wastewater management decisions. Using examples from the MANAGE* watershed assessment method developed for use in Rhode Island (Kellogg, 1997), we will outline key elements in screening-level assessment of small watersheds using computer generated maps. These assessment elements have evolved based our experience in conducting numerous watershed assessments as part of URI Cooperative Extension's educational and technical assistance to Rhode Island communities. The methods used incorporate recent research findings and well accepted modeling practices. Considered separately no one part of the assessment method is unique. But collectively the indicators and mapping generated, integrated with public outreach, form the ingredients of a practical, relatively low-cost tool that communities can adapt to suit local landscape characteristics and available GIS databases. Rhode Island communities are using assessment results to develop on-site wastewater management programs, including watershed-based performance standards using advanced on-site treatment systems in critical areas.
Assessment Approach
Our approach is both a nonpoint source model and a process of involving local decision
makers in the assessment leading to selection of locally-acceptable management practices. We
use a loosely-coupled Geographic Information System (GIS) model to collect soils and land
use data which is then entered into an Excel spreadsheet to generate nutrient loading and
watershed statistics. ESRI ArcView GIS software is used to spatially analyze and display
results. Key features of this decision support system include:
* Active local participation in each step of the assessment process builds on local knowledge
and integrates assessment with public outreach.
* Data scales relevant to local decision making, with a focus on small local watersheds and
wellheads. Study area size typically ranges from about 50 to 500 acres.
* Site-specific pollution "hotspot" mapping targets high risk areas for management or
additional data gathering.
* Risk indicators based on soils and land use features link land management decisions to
general water body health. Nutrient loading is used as one measure of pollution risk.
* Future alternatives analysis integrates cumulative impacts and provides a "big picture" view
of the consequences of wastewater management decisions on a particular water resource using
analysis of future land use (build-out) and pollution control options.
* Results focus on locally acceptable management practices that use available and accepted
technology implementable by local governments. Follow up assistance in adopting
management practices is offered and sources of additional support identified.
The assessment is designed to illustrate the cumulative effects of land use decisions and pollution management practices on local water resources. As a screening-level decision support system, MANAGE generates site-specific information needed to identify data gaps and guide management actions. Because the approach incorporates soil features used in evaluating site suitability for on-site wastewater treatment, and the scale of the land use analysis distinguishes between sewered and unsewered areas, the analysis is particularly useful in conducting needs assessment for wastewater management planning.
Functions
The method does not generate new data but instead, synthesizes existing information from
various sources to create a comprehensive risk assessment. MANAGE performs the following
functions:
1. Identifies pollution "hot-spots" ( areas where natural features and high-intensity land uses
together increase risk of nutrient movement to aquifers and surface waters.
2. Evaluates the effect of land use patterns on freshwater lakes and wetlands, groundwater
aquifers, and coastal embayments. The impact of future development on risks to water quality
can be explored based on current zoning or other future land use.
3. Compares effectiveness of storm water and wastewater management practices in reducing
pollution risk. A number of management scenarios can be tested for any portion of the
watershed to include, for example, improved septic system maintenance, sewering,
installing/retrofitting septic systems with denitrifying technologies, reduced imperviousness
with new development, improved storm water management, and fertilizer management on
either home lawns or agricultural lands.
Sources, Verification and Enhancement of Land Use Data
The RI Geographic Information System (RIGIS) provides the primary source of current and readily available data for the analysis, with the Environmental Systems Research Institutes' (ESRI) PC ArcInfo and ArcView GIS software. Our principal coverages used are the Rhode Island SSURGO (1:15,840) soil geographic database (USDA-NRCS, 1995) coupled with the 1995 Rhode Island Land Cover data (1:24,000, Andersen Level III system; Anderson et al., 1976). The SSURGO data base represents soil maps at the soil series level and includes attribute data for each major soil layer as well as use and management data. We combine soil types using four hydrologic groups representing ability of the soil to allow infiltration. Soil types are mapped by these groups, in combination with water table depth and presence of restrictive layers.
The land cover data consists of 38 different cover types and has a 0.2 ha minimum
classification size. These are lumped into 22 land use types for purposes of assigning runoff
and pollutant loadings. These are further grouped in map display for the sake of simplicity.
Percent impervious area associated with land use cover is derived from standard methods
(USDA Soil Conservation Service, 1975) and used in combination with soil hydrologic groups
to estimate the potential for surface runoff or groundwater infiltration on an average annual
basis. The land use coverage also provides an initial estimate of land use density and number
of housing units. This estimate is refined using local data. For example:
* Current land use data is updated using local input, US Census, and other sources.
* Build-out analysis is performed using zoning or other future land use map.
* Parcel based analysis is conducted where digital maps are available.
Sewer lines, hydrography (stream and pond) coverages, and watershed or recharge area boundaries complete the minimum data inputs necessary for the assessment. A wide variety of other natural features coverages and known pollution sources are selected for use based on the characteristics of the study area and the Town's objectives. Where data is available on land ownership parcels, a much more detailed wastewater needs assessment is possible. For example, additional map analysis might incorporate: dwelling units pre-dating adoption of 1970 State minimum individual sewage disposal system standards; permits issued for new systems and repairs; rental, seasonal and other high occupancy systems; commercial or institutional systems with high-flow or high-strength waste; known failures; dwellings with high water use based on public water supply records; and open or underdeveloped lots potentially suitable for shared leachfields.
Analysis Method And Products
The model is not directly linked to GIS software. Instead land use and soils information are extracted using ArcInfo and then brought into an Excel spreadsheet for analysis using common data files. The spreadsheet calculates a hydrologic budget and nutrient loading and summarizes watershed characteristics as indicators of watershed health. ArcView GIS software is also used to analyze pollution sources and display results. Results are presented in both map and chart form. A variety of specialized analyses can be conducted, based on resource protection priorities and availability of local data, such as parcel coverages.
MANAGE generates three types of assessment results: 1) pollution source "hotspot" mapping -a rapid screening of potential high-risk areas; 2) "watershed indicators" - measures of generalized ecosystem health based on soil and land use characteristics such as percent impervious area and percent forest cover. 3) A nutrient loading component incorporates results of current research on sources and fate of nitrogen conducted in coastal watersheds as one measure of pollution risk.
Hot Spot Mapping. All three types of output are designed to assist town officials to target limited resources to control the most serious pollution risks. But the hotspot mapping is most effective in allowing decision-makers to visualize problem sites. We start with the basic premise that high-intensity land uses are more likely to generate pollutants than less intensive uses. The risk that pollutants generated will actually reach groundwater or flow into nearby streams and ponds depends on soils and proximity to receiving waters.
Using ArcView software MANAGE locates high-infiltration areas and runoff-generating soils using soil permeability and water table depth. These hydrologically active soils reveal pathways of water and pollutant movement via either groundwater recharge or by direct flow through surface drainage networks. High water table soils are not isolated in the landscape but instead, are connected to small headwater streams and wetlands in an extended drainage network. Mapping this drainage network can help local officials identify natural functions to protect riparian areas where pollutant removal function is naturally high, and to maintain infiltration in recharge areas. High intensity land use where pollutants are most likely to be generated are then mapped. This includes commercial, industrial, transportation, institutional, active cropland, and high density residential land uses.
The MANAGE hotspot analysis combines these high-intensity land use with high-risk soil features for rapid screening of hotspots where pollutants are typically generated and where offsite movement is also likely. Various types of spatial relationships can be considered. For example, high intensity land use can be overlaid with high water table soils to reveal potential hotspots for storm water runoff and septic system failure. Other map analysis can highlight risks to groundwater in excessively permeable soils, unvegetated buffers, and erodible soils. Targeting hotspots efficiently narrows down potential problem areas for follow-up field investigation focused public education, or pollution remediation. This natural features mapping is also useful in evaluating site suitability for future development and in identifying pollution risks with future development. For future risk analysis, developable land is identified through "build-out" analysis using zoning or other future land use scenario.
Nutrient Loading to groundwater recharge and surface runoff is used as one measure of pollution risk. The Excel spreadsheet generates (a) nitrogen and phosphorus loading to surface waters such as reservoirs, lakes, and coastal waters from a watershed; and (b) nitrogen loading to groundwater from an area contributing to a groundwater reservoir or aquifer, such as a recharge area or wellhead protection area. The nutrient loading component of MANAGE is a watershed scale model, where average annual nutrient contributions from different land use /soil combinations within the watershed are estimated and compiled. We assign runoff and pollutant loading coefficients to 22-land use categories by four hydrologic soil groups. The basic assumptions used in the spreadsheet are consistent with those used in several widely accepted nutrient loading models. The spreadsheet has been customized for watersheds in Rhode Island, incorporating research on sources and movement of land use pollutants conducted in Rhode Island and other hydrogeologically similar areas. In addition, the spreadsheet estimates the number of hydraulically malfunctioning septic systems based on location in high water table and slowly permeable "restrictive" soils, with pollutant delivery based on proximity to surface waters. The nutrient loading results are most useful in comparing relative change in pollution risk under various land use and pollution control scenarios. The nutrient concentrations calculated for the groundwater recharge percolate and the inland surface waters should not be expected to match monitored well or surface water samples. Water reaching municipal wells represents a mix of various travel times and sources. In addition, estimates are based on pollution sources in the watershed, without taking into account potential nitrogen removal in wetland soils. Likewise, the average annual estimates do not take into account plumes of concentrated effluent.
Watershed Indicators summarize key information about land use and landscape features to evaluate general watershed health and predict the effect of these watershed features on aquatic ecosystems. Examples used are consistent with EPA indicators and include for example, percentage of high intensity land use, high water table and erodible soils, and estimated impervious surfaces in the watershed. These risk factors link general water body "health" with landscape features and management practices of the drainage area. They point to where streams, lakes, aquifers, and coastal areas are most likely to be at risk from land use activities. Indicators can also be used to evaluate potential impacts of watershed management practices by comparing, for example, the change in impervious cover with future land use or alternative management practices. In addition, indicators can be used to track actual changes in watershed conditions and potential risks over time.
Applying Assessment Results
Assessment output is designed to provide screening-level information needed to identify data
gaps and direct watershed management decisions. Results can be used to support a variety of
follow-up actions to include:
* Initiate, expand, or redesign water quality monitoring programs. Monitoring can focus on
volunteer shoreline surveys, vegetation mapping, flow studies, and baseline sampling.
* Public education on value of local resources, pollution threats and pollution prevention
actions for residents and businesses.
* Set priorities for site inspection of potential pollution hot spots to verify actual threats and
select a course of action.
* Site planning for new development, using natural features mapping to evaluate and avoid
impacts.
* Target stream buffers for restoration.
* Document need for wastewater management programs and other pollution control
actions.
* Develop performance standards for on-site wastewater treatment.
* Incorporate findings to support Town wastewater management plans and groundwater
overlay zones.
Summary
GIS-based watershed assessment has proven useful as a screening-level tool to identify data gaps and direct watershed management decisions. The MANAGE method is readily adaptable by communities with access to GIS software and four basic coverages: land use, soils, sewer districts, and watershed or recharge area boundaries. Technical expertise in GIS, hydrology, and watershed management is required to apply the model, interpret results and guide selection of management practices. Applying the assessment as a partnership between local communities and Cooperative Extension has been successful in meeting this need for technical support. For local officials ready to take actions to protect local watershed, assistance is available through supporting programs such as the URI On-site Wastewater Training Center and the RI Home*A*Syst residential pollution prevention program.
Rhode Island communities are using results of GIS-based watershed assessment to strengthen groundwater aquifer protection zoning, to support adoption of wastewater management ordinances, to build public awareness of water resource management issues, to draft commercial development standards, and to weigh rezoning decisions in aquifer recharge areas. The assessment provides local officials with site-specific information on local resources and pollution risks that are directly useful for decision making. Integrating community involvement in the assessment process can galvanize support for protecting local resources from a broad sectors of the community. Local officials' ability to take action in the face of uncertainty is one factor that affects the usefulness of risk-based assessment results. Community leaders have different tolerances for pollution risk based on a host of factors, including the value of the resource and evidence that potential threats will become actual sources of contamination. But as a screening tool, assessments provide a basis for further action even if the next step is targeted water quality monitoring or verification of identified threats. Follow-up assistance is important in linking assessment results to protection actions, especially in small communities with limited staff and resources.
Acknowledgements
Development of the MANAGE GIS-based watershed assessment was supported by grants from the Rhode Island Aqua Fund Program and the U.S. Environmental Protection Agency Region 1. Outreach and educational efforts were funded by the USDA Cooperative State Research Education Extension Service, Project #97-EWQI-1-0098. This paper is a contribution of the Rhode Island Cooperative Extension Service.
Author Biography
Lorraine Joubert is coordinator of the Cooperative Extension Municipal Watershed Training Program; James Lucht is an environmental planner and GIS specialist within this Extension program. Dr. Arthur Gold is a professor of watershed hydrology and science advisory on this project. Dorothy Q. Kellogg, currently a research assistant investigating role of riparian buffers in denitrification with Dr. Gold, is the primary author of the MANAGE assessment method. All authors are in the Department of Natural Resources Science at the University of Rhode Island.
References
Anderson, J.R., Hardy, E.E., Roach, J.T., and Witmer, R.E. 1976. A Land Use and Land Cover Classification System for Use with Remote Sensor Data. U.S. Geological Survey, Professional paper 964. Reston, VA.
Joubert. L., D.Q. Kellogg, A.J. Gold, and A. Mandeville. 1996. Water Quality Impacts of Changing Land Use on Block Island, Rhode Island. University of Rhode Island Cooperative Extension. Kingston, RI.
Kellogg, D.Q., L. Joubert, and A.J. Gold. 1997. MANAGE: A Method for Assessment, Nutrient-loading, And Geographic Evaluation of Nonpoint Pollution. University of Rhode Island Cooperative Extension, Department of Natural Resources Science. Kingston, RI.
Rhode Island Geographic Information System. 1989. R.I. Department of Administration. Providence, RI. Based on Rector, D. 1981. Soil Survey of Rhode Island. Soil Conservation Service. Warwick, RI. (Also available from U.S.D.A. Natural Resources Conservation Service).
U.S.D.A. Soil Conservation Service. 1975. Urban Hydrology for Small Watersheds. Technical Release No. 55 (TR-55), Washington, DC.
This page created by Roland O.
Coburn
Reasearch Tech I
on 3/21/00.