New York Rural Water Association

INTRODUCTION Recently, New York Rural Water Association purchased Geographic Information System (GIS) software. As NYRWA's Source Protection/Ground Water Specialist, I have started utilizing GIS in my wellhead protection programs. I quickly realized that GIS is a very powerful tool, one that can be used by both NYRWA and its member systems. The purpose of this article is to explain the fundamentals of GIS and illustrate how it can be used to analyze spatial data. The example problem I have chosen involves identifying areas that have the risk of nitrate ground water contamination. As indicated in previous articles, I believe that nitrate ground water contamination is one of the most prevalent threats to the quality of drinking water in rural New York State. DEFINITION What exactly is GIS? ESRI, a leading manufacturer of GIS software defines a geographic information system as "a method to visualize, manipulate, analyze, and display spatial data." Essentially GIS involves creation of smart maps, those that link databases to maps. GIS allows you to not only visualize data better through the production of nice looking maps, but it also helps you to analyze data and solve problems. This type of analysis is illustrated by the following example. EXAMPLE PROBLEM

Step 1: What are the questions to be answered?

Nitrate (NO3) is a ground water contaminant commonly associated with rural settings since it is derived from fertilizers, manure, and septic systems. Nitrate is very soluble and mobile in ground water. The first question to be answered through GIS is: What areas are at highest risk of nitrate contamination of ground water? The second question is: What areas are at highest risk of drinking water impact from potential nitrate contamination of ground water?

Step 2: What are the factors to be analyzed? Based upon my work with rural water systems over the past six years, nitrate contamination in ground water is most commonly found in areas with surficial sand and/or gravel deposits and row crops such as corn. In these areas, fertilizers and manure are applied to the exposed sand and/or gravel. These nutrients often leach into the permeable sand and gravel soils. Without a confining low permeability layer, the nitrogen fertilizers reach the underlying aquifer. Obviously, the potential impacts upon drinking water are greatest in those areas that rely upon ground water as a source of water supply. Thus, the data required to identify areas at risk of drinking water impact from nitrate ground water contamination are:
   surficial geology maps;
   land cover data; and
   water supply usage data. Step 3: Map areas of surficial sand and gravel deposits in New York State.

The best source of surficial geology data on a statewide basis is the New York State Surficial Geologic Map published by the New York State Geological Survey (NYSGS). The NYSGS has made this series of maps available in digital format. Using ESRI's ArcView GIS, I was able to generate a dataset of where sand and/or gravel is exposed at the land surface. These areas are depicted in Figure 1 below.

Step 4: Map areas of row crops in New York State.

Next, it was necessary to generate a GIS dataset that shows where row crops exist in New York State. The best available GIS data for land cover is the National Land Cover Data set (NLCD). The NLCD contains 21 categories of land cover information, including areas with row crops. The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, has produced the NLCD using Landsat thematic mapper imagery from around 1992 and supplemental data. Using ArcView GIS, I was able to select areas of row crops from the NLCD and map these separately. This is depicted in Figure 2 below.

Step 5: Map areas that have high nitrate ground water contamination potential. In order to define areas in New York State that have high nitrate ground water contamination potential, it was necessary to know what areas had surficial sand and/or gravel and row crops. Using ArcView GIS, I was able to intersect these two datasets and generate a new dataset of areas where surficial sand and/or gravel and row crops both exist. These areas of higher nitrate ground water contamination potential are depicted in Figure 3 below (is your area depicted on the map?).

Step 6: Identify counties with highest nitrate ground water contamination potential. Although Figure 3 above is very revealing and informative, it would be nice to know which counties had the greatest percentage of land with high nitrate ground water contamination potential. To does this, I used ArcView GIS to calculate the amount of area in each county that had both surficial sand and/or gravel and row crops. This leads to another dataset that is mapped in Figure 4 below. In this way, I was able to determine that the counties with the highest potential for nitrate groundwater contamination potential are: Columbia, Genesee, Livingston, Madison, Montgomery, Monroe, Oneida, Ontario, Orleans, Rensselaer, Suffolk, Washington, and Wyoming Counties.

Step 7: Identify the counties with the highest risk of drinking water impact from potential nitrate contamination of ground water. In order to measure the degree of potential impact upon drinking water supplies, it was necessary to first know the amount of ground water usage in each county. Using water usage data compiled by the United States Geological Survey, a new dataset on ground water use was generated. Figure 5 below is a map of ground water use by county in New York.

Note that some counties that had a high degree of risk for nitrate ground water contamination do not rely heavily upon ground water (e.g. Monroe County). Conversely, some counties with a more moderate nitrate ground water contamination potential are very heavily reliant upon ground water (e.g. Cortland County). The last map that was generated answers the question: What areas are at highest risk of drinking water impact from potential nitrate contamination of ground water? By multiplying the percentage of ground water use (Figure 5) by the percentage of the county with areas of high nitrate ground water potential (Figure 4), I was able to determine that the counties with the highest risk of drinking water impact from potential nitrate ground water contamination are: Columbia, Cortland, Genesee, Livingston, Steuben, Suffolk, and Wyoming Counties (see Figure 6 below).

Counties with moderate risk of drinking water impact from potential nitrate ground water contamination are: Allegany, Cattaraugus, Chautauqua, Chenango, Madison, Montgomery, Ontario, Orleans, Oswego, Rensselaer, Saratoga, Schoharie, Tioga, Washington, and Yates Counties (see Figure 6). FINAL THOUGHTS
Identifying areas at risk of contamination is just one of many uses of GIS. As an association, we are starting to use GIS to know better where our members are and are not. Using GIS we can better plan our site visits and target potential new members. As systems, you can use GIS to map your distribution system and effectively manage data on your system components. You can also use GIS to conduct analyses such as where leaks have and will likely occur. If you would like to learn more about GIS or nitrate contamination potential, please call me at 1-888-NYRURAL or e-mail me at Winkley@nyruralwater.org.