Ambiguity of the Definition and Delineation of the Groundwater Table

Ambiguity of the Definition and Delineation of the Groundwater Table 

Recently I attended a day-long course titled – Where’s the Water Table, or Water Table Training. It was taught by soil scientists and hydrologists and sponsored by the Pennsylvania Independent Oil & Gas Association (PIOGA). I learned that the water table can be defined in different ways depending on the reason or application for its delineation. I learned that water table can mean something slightly different to a soil scientist than to a groundwater hydrologist. Typically there is a single – zone of aeration – where the soil voids are occupied by both air and water and below that a single – zone of saturation – where the voids are occupied solely by water. From a groundwater hydrology textbook: 

“In the absence of overlying impermeable strata the water table, or phreatic surface, forms the upper surface of the zone of saturation.”

Water table is also defined as:

“… the level at which water stands in a well penetrating the aquifer.”

I learned that the water table can mean something different to those determining the suitability of different types of septic systems than to those determining the drainage, grading, and construction of a storm water management system, and again different than those determining the suitability of a site for construction of a gas well pad.

A seasonal water table is different than a permanent water table. While some soil scientists may refer to a seasonally high water table as a “perched water table” the term – perched water table – has a different meaning to a groundwater hydrologist who would note that a true perched water table is rare in nature and would be confined to a specific geological condition where an aquifer of limited areal extent is perched above the regional water table in that extent. Saturation can occur in different frequencies, sometimes in multiple seasons or even more frequently. There are also daily changes and typical reactions to rains due to recharge rates. Saturated conditions in different rocks and soils show different characteristics: hydraulic conductivity, groundwater flow rates, maybe different hydraulic gradients, different degrees of anisotropy, etc. Type of soil, in terms of composition and grain size distribution, and degree of saturation are the two most important ground conditions that affect construction projects. They are also two of the most important factors in assessing contaminant transport. Heavy rains can also create temporary, or perched water tables that can lead to slips or landslides.

Ideal determination of saturation conditions would require some degree of understanding saturation of the site throughout the year, type of soil, and the type of project. Detailed info could best be gathered by drilling test wells but excavated soil profiles are typically the main approach. Soil scientists would correlate colors (a key indicator of saturated conditions) and look for mottling (often in gray vertical streaks) and other redoximorphic features, those that indicate the reducing conditions brought about by newly saturated soil. Reducing reactions take place in chemical order and so degree of reduced chemicals can hint at how often and/or how much conditions are saturated. If there is sufficient organic matter present the sequence begins with aerobic decomposition by oxygen-consuming bacteria (aerobes). When saturated conditions and subsequent low oxygen conditions occur for long enough then anaerobic decomposition begins with reducing reactions in decomposition by anaerobes. The chemical sequence of reducing reactions begins with oxygen to nitrogen, to iron, to manganese, to sulfur, to carbon, and then to microbial gas. These constituents and their relative percentages can be clues to past saturation conditions. For instance a sulfur smelling soil indicates enough saturation and saturation-time to have reduced significant amounts of sulfur which is closer to generation of microbial gas, typically referred to as “swamp gas.” This indicates prolonged submergence.

    

Saturated conditions close to surface, whether seasonal or permanent, can create problems for construction, drainage, landslide potential, and be more easily contaminated by spills. Structures built on or partially in saturated soil can be subject to hydrostatic uplift pressures which is buoyancy derived from saturated pore water pressures. In order for structures such as underground tanks to avoid hydrostatic uplift they must be built heavy enough to withstand those pressures – typically 1.5 times the pore water pressure. Spills are easier to clean up in unsaturated soils where they can be excavated and the soil treated offsite. When contaminants enter groundwater they can be more mobile and much more difficult to remediate. Saturated clay soils are the most problematic as they swell when wet and shrink when dry but saturation can cause issues with other soils as well.

  

One of the issues at the training was that the newly proposed drilling regs in PA, recently voted down in the PA legislature, (as I understand it) had provisions requiring that unconventional well pads could not be graded down below the water table. Depending how one defines the water table, that could have unnecessarily eliminated many locations that have seasonal or cyclical saturation or uncertain saturation. Despite opposition from oil and gas companies as well as some environmental science and civil engineering companies, that designation was kept. Since the regs were rejected this is not likely to be a concern in the near-term but new regs could be revived with similar provisions.

  

References:

Groundwater Hydrology (Second Edition) – by David Keith Todd (Wiley and Sons, 1959, 1980)

Water Table Training – Where’s the Water Table – sponsored by Pennsylvania Independent Oil & Gas Association (PIOGA), Feb., 2016

Geotechnical Engineering: Principles and Practices – by Donald P. Coduto (Prentice-Hall, 1998)

Environmental Geology: Fourth Edition - by Edward A. Keller (Merrill Publishing, 1985)