Evaluating the vulnerability of bored and driven wells in a shallow unconfined aquifer.

Author:Schmalzried, Hans
Position::FEATURES - Report
 
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Introduction

Over 30 million people in the United States depend on private wells as their source of drinking water (Centers for Disease Control and Prevention [CDC], 2006). According to the 2005 American Housing Authority Report, 15,372,000 homes in the United States are served by an individual well (United States Census Bureau, 2005).

In some rural areas, shallow unconfined aquifers are the only source of water available for private wells. It is unknown exactly how many private wells are developed in shallow unconfined aquifers. Shallow wells are generally developed in sand and gravel aquifers that are characterized by large pore size, which allows rapid and easy passage of both water and contaminants (Verstraeten, Fetterman, Sebree, Meyer, & Bullen, 2004). Because of this characteristic, as well as the shallow depth, drinking water from shallow domestic wells is more vulnerable to surface-derived contaminants than drinking water from deep wells (Kelly & Wilson, 2002). The quality of water found in shallow aquifers can differ according to land use and impacts of human activity in the area. By-products of human activity in nonindustrial suburban and rural areas that were found to impact shallow well aquifers include road salt runoff, water softener brine waste, animal and human waste, and agricultural and lawn chemicals (Minnesota Pollution Control Agency, 2001).

Specific tracers have been identified as indicators of different human activities in groundwater. In agricultural areas, pesticide, herbicide, and fertilizer concentrations are used as tracers to evaluate contamination. Results from 41 land use studies found low levels of pesticides in shallow groundwater in both urban and agricultural settings (Kolpin, Barbash, & Gilliom, 2002).

In cold regions where roads are salted, sodium and chloride are markers that indicate roads and highways as sources of shallow well aquifer contamination. The presence of chloride in groundwater has been associated with other human activities including animal and human waste, fertilizers, and industrial products (Lundmark, 2005; Minnesota Pollution Control Agency, 2001). High total dissolved solid (TDS) levels in shallow groundwater can also be an indicator of road salt runoff and vehicular exhaust. Residents with water softeners and onsite wastewater treatment systems may find chloride and sodium ions in their drinking water (Kelly, 2005).

The presence of nitrogen species (i.e., ammonia as nitrogen, nitrate as nitrogen, nitrite as nitrogen) in groundwater is used as an indicator of septic system, animal waste, and fertilizer contamination. In shallow aquifers where the presence of oxygen is abundant, however, denitrification may limit its usefulness as an indicator (Verstraeten, Fetterman, Sebree, Meyer, & Bullen, 2004). Boron has been demonstrated to be a good indicator of septic tank inputs to groundwater. Major sources of boron in household waste include detergents and cleaning agents (LeBlanc, 1984; Minnesota Pollution Control Agency, 2001).

In shallow aquifer areas where options for a deep well are limited, bored and driven wells are commonly used for domestic water supplies. Bored wells are constructed with a bucket auger and utilize a concrete or large diameter (typically 36 inches) PVC pipe as the casing. The annular space between the surrounding soil and the well casing, created during boring, is filled with mason sand due to the uniform fine particulate (grain) structure and desirable filtration process. These wells are installed where the existing natural soil, usually sand, is too shallow and does not provide a sufficient flow of water to allow construction of a driven well. A bored well creates a reservoir to collect and hold the water. Driven wells are constructed of a series of connected small diameter pipes fitted with a point on the end. The well point is driven into the ground by pounding or is washed in using high water pressure. Both of these well construction types have casings that are generally less than 25 feet (7.0 m) deep.

The Safe Drinking Water Act does not cover private well water supplies (Safe Drinking Water Act, 1974). Local health departments (LHDs) in many areas throughout the country administer regulations adopted by their state for private water supply systems. Public health regulations for private water supply systems are primarily limited to construction standards on new systems and alterations to existing systems. Because driven and bored wells are vulnerable to contamination, some states and LHDs have forbidden their use while others continue to allow them. As in other states, Ohio law allows local boards of health to issue variance permits for new or altered water supply systems that do not meet code (Ohio Revised Code, 1984). Variances from well casing length regulation are commonly approved to allow shallow water supply construction in areas where deep wells cannot be developed. While recommended well casing lengths vary according to the specific geology of an area (Chinn, 2003) most states require a minimum of 25 feet (7.0 m) of casing. To assure consistency in their approvals of variances, boards of health often establish policies for the issuance of commonly granted variances such as those allowing the development of short casing wells in shallow aquifers.

The Oak Openings Region in northwest Ohio provides an ideal area to study shallow wells constructed in an unconfined sand aquifer. Approximately 10,000 individuals in the area consume water from a 335 square...

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