All's Well That's Spaced Well

James Lawrence

As required by Subtitle D and state regulations governing municipal solid waste landfills, the design and installation of a groundwater monitoring system is necessary to evaluate if a landfill is impacting groundwater resources. Groundwater monitoring system design is based on an in-depth understanding of the site-specific hydrogeologic situation. Monitoring wells are used for periodic sampling and analysis of water samples. The resulting data are analyzed to determine if the landfill is impacting the environment.

The spacing between wells is a key design consideration for landfill monitoring systems. Well spacing requirements depend on a combination of hydrogeological and non-hydrogeological factors. Hydrogeological factors affecting well spacing include hydraulic conductivity (the extent to which soil allows water to move through it) and the speed at which groundwater flows beneath the site. A leachate plume released into slow-moving groundwater will generally disperse more widely as it travels downgradient, in contrast with a fast-moving plume. A flow net constructed using site data can help to determine appropriate well spacing.

Equally important are a range of non-hydrogeological factors. Factors to consider include:

Positioning wells near the lowest points in the landfill (i.e., leachate sumps)

Many monitoring systems are designed to position wells near the lowest points in the liner system, because leachate is more likely to exit at that lowest point. Because landfill leachate sumps are not generally positioned at equi-distant points around the landfill, wells placed to monitor such sumps will not be equally spaced around the landfill.

The position of pre-existing wells

If wells must be added to a pre-existing monitoring system to comply with new regulatory requirements, the existing network should be carefully considered and optimized. This will ensure continuity and reduce the cost of adding unnecessary monitoring wells.

The position of obstacles

A typical landfill property has numerous obstacles to monitoring well installation, and other areas that may allow well installation but are not suitable monitoring locations. Drainage ditches and ponds should be carefully considered before placing wells. It is easy to unintentionally monitor a drainage ditch or a pond on the property.

Addressing all of these design issues may reveal that an irregularly-spaced monitoring well design is more suitable than a system based on equal well spacing.

State regulations are another factor to consider. A survey of state regulatory entities was conducted to assess the well spacing requirements at municipal solid waste landfills across the United States. About 60 percent of the states responded. According to the survey, the vast majority of states maintain regulatory flexibility on well spacing. Nearly 77 percent of the states that responded have no formal or informal spacing requirement, 8 percent have an informal, expected maximum spacing, and only 15 percent of the respondents maintain a fixed maximum distance between monitoring wells.

Most states have a flexible written regulatory policy requiring a “sufficient number of wells” to obtain “representative groundwater samples.” Very few states require a prescribed distance (number of feet) between wells. Some solid waste state regulatory agencies use an informal distance guideline to assess monitoring designs submitted for regulatory approval.

Computer models with a broad range of complexity can be used to assess well spacing designs. Analytical models are relatively simple and assume single, site-wide constant values for inputs such as hydraulic conductivity, gradient and other factors. Numerical models are complex by comparison and require a distributed grid of input data that can be varied across the site. In many cases, a landfill's hydrogeological characterization may not have enough data to drive a numerical model. Also, some regulatory agencies may lack the time and expertise to evaluate such complex models.
— James Lawrence
Senior Hydrogeologist
SCS Engineers