The type and severity of water contamination often is directly related to human activity, which can be quantified in terms of the intensity and type of land use in the source areas of water to streams and aquifers. The analysis of patterns of land use and population provides a tool in the investigation of sites with known contamination, and in the prediction and prevention of future contamination of downstream waters. Studies of contamination sources and transport pathways that affect surface water and groundwater draw upon several disciplines, including hydrology, geology, biology, soil science, agriculture, physics, chemistry, and engineering.
Land use and land cover largely determine the type and amount of contaminants entering streams, lakes, and underground pathways, including aquifers. Some contaminants occur and move naturally (white arrows), whereas others are produced by human activities (hatched arrows), and their movement often is accelerated as a result of rainfall that accentuates runoff and infiltration.
A relatively simple way to study the effects of land use on groundwater quality is to compare the predominant land uses within a given area to the concentrations of selected contaminants in water drawn from shallow aquifers within that area. Analysis of the relation between land use and the magnitude of contamination in a specific area primarily is based on the following two assumptions.
First, it is assumed that contaminated groundwater at a well originated as uncontaminated recharge (precipitation) that passed through a contaminated area before reaching the well. The area from which a well derives its water (and associated contaminants) is known as the well’s groundwater “contributing area.” A well’s contributing area can be delineated on a map.
Second, it is assumed that the contaminants detected in groundwater were present within the well’s contributing area and were transported by groundwater flow to the well. The source(s) of contaminants within a contributing area, such as buried septic systems and leaking underground fuel tanks, can be difficult to identify and locate. In many instances, these sources can be inferred from the type and intensity of land use within the contributing area.
Soil erosion and water runoff from cropland into nearby streams can be a major source of sediment, nutrients, and pesticides in watersheds dominated by agricultural land. This photograph shows poor cropland management in which the tilled field extends to the edge of an unvegetated (and eroding) streambank. Implementation of soil and water conservation measures, such as buffer strips of undisturbed land between cropland and adjacent streams, can provide an effective control that reduces contaminant entry into aquatic systems.
Row crop agricultural is the predominant land use in the Upper Iroquois Watershed. Agricultural land use accounts for more than 84% of the watershed. The Iroquois watershed was predominantly marsh and wetlands with interspersed islands prior to the area being settled. Over the years, human kind has cleared the land and created an extensive system of open ditches and subsurface drainage tile for the purpose of agricultural crop production. This change in land use has resulted in the loss of wetland function, groundwater recharge, and flood storage.
Forest and developed/urban lands are the next largest land use within the watershed. Each accounts for a little more than 6 percent of the watershed. Developed areas are generally concentrated in and around cities and towns within the watershed. However, in recent years there has been a trend of residential strip development taking place along county and state roads. This type of development requires more individual wells and on-site septic systems which can have a greater effect on water quality verses public/communal wells and sewage treatment plants which allow greater flexibility in the control of pollutants.
Hay/pasture, grasslands and shrubs make up another 3% of the watershed. The remainder of the watershed consists of isolated pockets of wetlands and open water. There are no large lakes within the watershed. Most open water areas consist of man-made ponds, water-filled quarries, and backwater areas which are scattered throughout the watershed.