ABSTRACT: In 1986, amendments to the federal Safe Drinking Water Act established a Wellhead Protection Program to protect wells and well fields supplying public water from contamination. Two key elements within this plan are the definition of wellhead protection areas (WHPAs) and determining potential sources of groundwater contamination within the WHPA. These WHPAs serve as an educational tool to inform citizens of activities that affect their drinking water supplies. The objective of this project is to investigate the feasibility of using a GIS for WHPA delineation and contaminant source identification on a statewide basis. Twenty-five wellheads are selected for delineating wellhead protection areas in this pilot project. For each WHPA, a GIS layer is generated from orthophotos using a limited set of land use categories. A series of base maps are produced which will allow persons in each community to perform a ground survey. Highly detailed information will be gathered at the local level and placed on the maps. These maps will then be used to update the GIS layer which can be used to determine potential threats to public drinking water within the area.
INTRODUCTION
In 1986, amendments to the federal Safe Drinking Water Act established a Wellhead Protection Program to protect wells and well fields supplying public water from contamination. Each state is required to submit a Wellhead Protection Program to the Environmental Protection Agency (EPA). The Missouri Wellhead Protection Program contains a series of steps for implementing a local Wellhead Protection Program for those interested in protecting a drinking water resource. Two key elements within this plan are the definition of wellhead protection areas (WHPAs) and the determination of potential sources of groundwater contamination within the WHPA. The Center for Agricultural, Resource and Environmental Systems (CARES) was contracted by the Missouri Department of Natural Resources to define WHPAs for several public water supplies (PWS) in Missouri and to develop maps showing the potential sources of contamination within those WHPAs. A geographic information system (GIS) was used to calculate the WHPAs and develop the land use maps for each PWS. These land use maps will be used to determine potential sources of groundwater contamination. The objective was to outline a methodology for inventorying potential sources of groundwater contamination which could be applied to any public water supply in the state.
PILOT STUDY
The methodology used for this project is based on a pilot project performed during the spring of 1994. In this original project, methods for delineating a WHPA were investigated. These included using a fixed radius, using a calculated radius, and using a semi-analytical model designed specifically for delineating WHPAs. The model required data which was unavailable without a detailed hydrogeologic survey of the area and was therefore not recommend to be used on a statewide basis. The calculated radius method based on pumping rates was applicable, but unpredictability in groundwater flow paths and possible future changes in pumping rates also precluded the use of this method. The fixed radius method, while leaving much to be desired, seemed the best approach to WHPA delineation. The main problem to with using this method was determining what size radius was appropriate. A one-half mile radius was used in the pilot project.
Different approaches for compiling a contaminant source inventory within a WHPA were also investigated. One idea involved using 1:24,000 scale USGS topographic maps as a base and having local volunteers draw the various potential sources of contamination on the map by hand. It was quickly determined that the scale of these maps was not large enough to provide the necessary detail for the inventory. A second drawback was that the quadrangles were often thirty or more years out of date and therefore did not include many of the recent streets and buildings constructed in the area.
An alternative approach was to use the Missouri Department of Highways and Transportation city street maps as a base map upon which to draw the land use within a WHPA. The contamination source maps would be developed by driving through the WHPA and mapping what was seen. The city street maps turned out to be too large to be easily handled and did not provide a base for areas of the WHPA outside the city limits. Also, a basic problem with a driving survey of the WHPA was that not all areas of the WHPA were easily accessible, especially in the areas outside the city limits.
The third approach was to purchase the county assessor maps for the study area and use the orthophoto base on these maps to determine the land use. These maps, available at varying scales (1:1200, 1:2400 and 1:4800), have the level of detail necessary for mapping the land use activities which might affect groundwater. This approach worked well for the limited number of land use categories used in the pilot study. Therefore, the study indicated that using a simple method for determining the WHPA (fixed radius) and using detailed aerial photos for the contamination source inventory (assessor maps) was the most practical method for use within a statewide wellhead protection program.
CURRENT PROJECT
In September of 1994 CARES was contacted about a project to use the methods developed in the pilot study for several public water supplies which were interested in implementing a wellhead protection program. CARES agreed to develop WHPA boundaries and contaminant source inventories for a minimum of twenty five wells and to provide maps to the PWSs for each WHPA developed. A list of public water supplies was agreed upon by the Missouri Department of Natural Resources and the Missouri Rural Water Commission. This list contained the cities most interested in the wellhead protection program.
An overall outline for this project was developed and has been refined during the project. This outline is as follows:
1) Create the WHPA boundaries for each water supply 2) Determine which county assessor maps were needed to map this area, and order the maps 3) Develop the land use categories for the contaminant inventory 4) Scan the maps 5) Georeference the images 6) Photo-interpret the images 7) Create maps for each WHPA 8) Create work maps for a ground reconnaissance
The first step in the process was to construct the WHPA around each well within each system. The fixed radius method was used to define the WHPA boundaries. The one-half mile radius used in the pilot study was increased to a one mile radius. This was done to insure that the area affecting the wellhead was within the area actually mapped. The GIS was used to construct these boundaries. Once this was done, the maps covering the WHPA could be ordered from the county assessor offices.
As the maps began to arrive from the counties, several differences between the pilot project maps and these maps were noticed. First, not all of the maps were ortho- rectified, meaning they had not been corrected for distortions. Second, unlike the maps used for the pilot study, these maps did not always have a coordinate grid which could be used to georeference the photo. Finally, the quality of the maps varied greatly from county to county. These factors would have to be dealt with as part of the process of creating the contamination source inventory maps.
While the maps were arriving, a list of land use categories was developed for use during the photo-interpretation phase of the project. A list of over 100 land uses in thirteen different categories was developed. This list was based on a list of potential contamination sources and activities from the Missouri Wellhead Protection Program. Each land use was assigned a code for entry into the GIS.
At this point the process departed from that developed during the pilot project. Instead of interpreting land use directly on the county assessor maps and then retracing the line work on the digitizer, the maps were scanned into the computer so that an on screen, or 'heads up', digitizing process could take place. This process has several advantages. Each line is created only once on the screen, rather than drawing the line on the map and then retracing it with the digitizer. Additionally, the images can be georeferenced before digitizing takes place, thereby eliminating the process of edge matching many separate layers into one large layer. Also, the creation of data could take place on any computer, not just the ones attached to a digitizer. The major drawback is that the scanning process creates large files (20-90 Mb) which must be manipulated by the computer. This required the use of computers with a large amount of RAM (>32 Mb), a suitable graphics adapter and a fast processor.
Once the county assessor maps were scanned, it was necessary to compensate for the inconsistencies in quality. The images were altered using image processing software to lighten or darken the images and to increase the contrast between features. This process provided us a more consistent image base. This process did not alter the shape of the image, only the graytone quality.
The next process was to georeference the imagery. Because many of the images did not have coordinate grids, significant features were used for georeferencing. These features were located on a 7.5 minute quadrangle and digitized into the GIS as point features. Those points were then used to rectify the image and reference the image to a coordinate system. It should be noted that by using this process, even though the imagery has a level of detail at scales of 1:1200, 1:2400 and 1:4800, the resulting positional accuracy is reduced to a 1:24,000 scale level.
The photo-interpretation phase is the most important and time consuming phase of the project. During this process, an image is displayed in the background while the photo interpreter draws lines into a data layer overlaying the image. As each area is outlined, it is coded with the appropriate four digit land use code. The finished product is a layer cataloging the land use throughout the WHPA.
Two final items are necessary to make the information usable at a local level. First, a large, full color wall map depicting the WHPA is provided to the public water supply. This map can be used to show the citizens of the community the activities in the community which might affect their drinking water. This will serve to raise awareness in the community about groundwater issues. The second product is a standard series of maps to be used by local volunteers for a ground survey of the WHPA. This is necessary because not all activities which can affect groundwater can be determined from aerial photography. The maps will also allow for the recording of land use changes in the WHPA which have taken place since the date of photography. The maps are at a 1:2400 scale which will allow volunteers to easily locate features within the WHPA and update the features as necessary. Each area on the map is numbered with its current land use code. These maps are bound into a 12 by 22 inch booklet with a map sheet index and a list of the possible land use codes.
RESULTS AND RECOMMENDATIONS
The use of the county assessor maps for generating land use remains a less than satisfactory process. When the pilot study was performed, the assessor maps were of high quality, up to date and had a coordinate grid for georeferencing. The assessor maps obtained for this study did not always possess these traits. A major problem was the quality of the information received from the county assessor offices. The assessor maps are sent as blueprint reproductions of actual photographs. It appeared the quality of the blueprint was strongly dependent on the quality of the blueprint capabilities of each county. The amount of variability in the quality of the photo base caused much time to be spent on the poor quality photos before photo- interpretation could take place. For the large images, the image processing time could take in excess of one hour. The images which were of high quality required no additional image quality processing after scanning.
A second problem with the assessor maps is that most do not have a coordinate grid for georeferencing. Additional time had to be spent in locating features which appeared on both the assessor maps and the 1:24,000 USGS quadrangles for registration. Because of the difference in scale, the image rectification process was not always uniform. This would sometimes require the locating of additional points and a re- registration process. Considerable time could be spent on this step. Even with careful registration, it was sometimes necessary to warp the finished layer to insure at least a 1:24,000 scale level of quality. Again, this was done by registering features in the layer to features on a 1:24,000 USGS quadrangle.
The age of the assessor maps for many areas of the state is another problem. The date of the photography on the maps received for photo-interpretation ranged from 1980 to 1994. Many of the counties with fewer resources and slower growth rates have not updated the photography since the early 1980s. This means that the contamination source inventory maps for those WHPAs are up to 15 years out of date. This factor will make the ground survey of those areas more time consuming and difficult. Also, these older photos were usually of poorer quality.
The process of using the 'heads up' digitizing versus using the standard method of digitizing shows a clear time savings and quality control advantage over hand drawn interpretation and digitizing. Additionally, this process allows all material used for the process to be stored digitally and accessed again at a later date if necessary. This technique also helps to insure that ground features are not missed by displaying the vectorized information over the image and performing a visual check. However, when all the image processing and registration problems are introduced, the advantage of scanning the assessor maps is reduced. During the pilot project, it was estimated that the process of generating the WHPA boundary and land use layers took about 20-25 hours per square mile (not including time for the ground reconnaissance effort). The time required to reach the same point using the scanning and heads up digitizing process varied from 5 to 30 hours per square mile. The additional variance is due to the irregular quality of the assessor maps. However, it should be noted that if the pilot project had been performed with the poor quality maps used for some WHPAs in this study, the time per square mile would have also increased for the hand digitized method.
A major problem, therefore, is the quality and age of the assessor maps used for generating land use. Alternate sources for land use determination were again investigated. ASCS aerial photos were considered, but rejected. They are not ortho-rectified and do not have a coordinate grid for georeferencing. Also, the resolution of the photos is not as great as the assessor maps.
Sample Digital Orthophoto Quadrangles (DOQs) were obtained from USGS for Olmsted County, MN, and compared to the assessor maps. The advantages of using DOQs are apparent. They are already ortho-rectified and do not need georeferencing. Image quality is also high and does not vary as the assessor maps do. Additionally, they are more up to date than many of the assessor maps. However, the resolution of the DOQs is not as good as the scanned assessor maps. Small features such as parking lot stripes and storage tanks cannot be determined from the DOQs. Since this is primarily a problem in urban areas, the DOQs would be more than adequate for the rural areas. Unfortunately, most of the WHPAs are largely based in developed areas. A second factor in not using the DOQs is that they are not available for the majority of Missouri, nor will they be for a number of years.
Another alternative is to have aerial photos taken of each WHPA for use in the process of creating the contamination source inventory. Ideally, these photos would be taken with a digital camera and ortho-rectified for use in an on screen digitizing process. This would eliminate the time required for scanning, image processing and georeferencing. It would also provide current land use for the WHPA. However, this is a very expensive process. Currently, the county assessor maps are the most detailed and cost effective products available for all areas of the state at the resolution necessary for the contamination source inventory.
Once the contamination source maps have been produced, community involvement is needed to finish the inventory. This is required because the assessor maps still cannot show which buildings are paint stores or the locations of underground storage tanks, nor can they show the land use changes that may have taken place since the date of photography. For these it becomes necessary to develop an easy to implement methodology for ground reconnaissance. This would involve the production of maps which are easy to read and use in the field, and writing guidelines which are easily understood and followed by the reconnaissance volunteers. To this end, a standard series of maps has been produced for each WHPA in the study.
The result of this study is a methodology which can be used for any public water supply interested in wellhead protection. The study illustrated that there are economically viable resources available for determining land use within a WHPA available for any area in the state. It also demonstrated the effect data quality can have on a production schedule. The need for cooperation between local persons interested in wellhead protection and a guiding central agency to develop land use maps of the WHPA in order to provide consistent and accurate data is emphasized by this project. Finally, a product to facilitate this communication has been developed in the form of the map booklet and a large, full color map showing the land use in the WHPA. It is hoped that these products will serve to educate the population within a WHPA about how their activities can affect their drinking water. It is also hoped that these products can be used to interest other water supplies to implement a wellhead protection plan of their own.
Barnett, C. J. and S. J. Vance. 1994. Feasibility Investigation for Wellhead Protection Area Delineation. Research Report to the Missouri Department of Natural Resources.
Missouri Department of Natural Resources, Public Drinking Water Program. 1994. Missouri Wellhead Protection Program. Missouri Department of Natural Resources.
National Research Council. 1993. Ground Water Vulnerability Assessment. National Academy Press, Washington, D.C.