Interrelationships of Land Use, Soil Conditions and Groundwater Contamination Near Lindsay, Ontario

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Abstract
Surficial Quaternary sediments and weathered bedrock are an essential source of potable water for many rural communities throughout Canada. Due to their shallow disposition, they are highly susceptible to contamination by point and non-point sources. Point sources of contamination such as landfills and salt piles can cause severe degradation of water quality but generally these are of only local concern. Non-point contaminants such as nitrate fertilizer and chemical products of urban development can result in sudden and widespread pollution of groundwater following long-term accumulation and migration in the shallow subsurface. Most studies of polluted well waters tend to focus on individual wells or groups of wells and fail to reveal the nature and type of contaminant source.A regional hydrochemical study was conducted in a typical farming community near Lindsay, Ontario where some residents had expressed disquiet over excessive levels of chloride, nitrate and sodium in groundwaters from their shallow private wells. The study revealed that the shallow limestone bedrock interacts hydraulically with the shallow overburden to form a single integrated system which is contaminated primarily by pollutants from two non-point sources. Road de-icing chemicals were recognized as the major source of elevated chloride, the contamination being most significant on the down-flow side of major salted highways. Cation exchange has taken place in the contaminated waters. This confirms that the road salts are being transported by subsurface migration and that the contamination is not the result of overland flow and leakage into poorly sealed wells as is commonly supposed. By comparison, the nitrate contamination is widespread, its distribution being closely associated with well drained soils supporting fertilized agricultural land. Significantly, nitrate contamination is less severe in groundwaters underlying soils classified as imperfectly or poorly drained, even though in some areas these soils support similar categories of land use. The approach described clearly demonstrates how a relatively simple and inexpensive regional survey of groundwater chemistry can yield valuable information on the aquifer system and on the nature, distributions and origins of the groundwater contamination.