Groundwater & Pesticides -- Section 2.3 Understanding the Water Below

A general understanding of water's journey through the ground can and should inform public debate about the policies and management programs to prevent pollution from pesticides and other contaminants.

Consider again the sponge analogy. The sponge -- or the earth in this case -- is not of uniform consistency. It has many layers, laid out in a complex pattern.

Water is held within a given layer or geologic formation. The characteristics of that layer and its relationship to other layers determine some of the water's characteristics -- its abundance, its ability to move, and often its chemical makeup. Those layers or formations that hold and transmit water sufficient to yield a usable water supply are called "aquifers."

While the standard diagram of groundwater flow shows layer-cake formations of several homogenous rock formations, things are rarely so simple in nature. Geologic formations are seldom deposited uniformly; they may be highly complex, folded, and/or fractured.

Diagram from Baldwin, Helene L. and C. L. McGuinness, U.S. Geological Survey, A Primer on Ground Water, 1963.

Small pockets of water-bearing strata may be interspersed with non-water-yielding formations. Or similar materials -- comprising a large interconnected aquifer -- may stretch over vast regions. Water moves over areas in nine states through the Dakota Sandstone, for example. Aquifers can be found a few feet or many hundreds of feet below the land surface. Vertically they can extend for a few feet or hundreds of feet.

Generally, aquifers are classified into two broad categories. "Unconsolidated aquifers" consist of relatively loose particles of sand, gravel, rock debris, weathered bedrock and soil. In these areas, water -- like water in a gravel-filled fish bowl -- is held in the open spaces or pores that lie between the particles.

"Consolidated" deposits or rock masses may also serve as water-bearing strata. Water in these areas, which may be comprised of rock types like sandstone, granite, limestone or shale, occurs in cracks, fractures and solution channels. The size of these openings and the degree to which they are interconnected can vary dramatically and will determine the extent to which the formation may serve as an aquifer.

Two terms are important but often confused. The amount of open space available to hold water in a formation dictates the "porosity" or storage capacity of an aquifer. "Permeability" -- the aquifer's ability to transmit that stored water -- is a function of both the volume of the open spaces and the extent to which they are interconnected.

Formations with very small and poorly connected open space yield little water and generally restrict the flow of water. These "aquitards" or "confining layers," like aquifer formations, can be small or extensive. Where an aquifer is sandwiched between two aquitards, it is often called a "confined aquifer."

As is the case with the terminology for groundwater and surface water, the language related to aquifers implies a straightforward and clear distinction between aquifers and aquitards and between confined and unconfined aquifers. Reality offers more of a continuum than a simple, bright dividing line. Geologic formations may be more or less permeable, more or less protected from pollution from adjoining or overlying layers.

Time is an important additional dimension -- many formations considered "impermeable" will, in fact, allow for groundwater movement over long periods of time. The average rate of groundwater movement through coarse sand, for example, may be about 360 feet per year. Through clay, groundwater may move less than one-half an inch per year. Groundwater that lies half a mile below the land surface may easily be 200 years old; in very deep aquifers, the groundwater's "age" may be measured in thousands of years.

One basic rule is: Don't believe a general statement in hydrology because groundwater is such an incredibly diverse entity that there is a great range of properties that are present there. ... The statement that groundwater moves slowly through porous media is reasonably correct for a majority of the cases in hydrology: 60 percent, maybe even 70 percent. ... The assumption that the professional modelers make is that aquifers are homogeneous in all directions; it just isn't true.

Dr. E. Calvin Alexander ¹

This generally slow movement of groundwater, though it might imply a long lead time for dealing with problems, too often means long-term and difficult-to-manage impacts from pollution. It can be difficult, if not impossible, to clean that sponge that you cannot see and you cannot squeeze.

Moving Downward

Before water or other liquid reaches areas in the ground fully saturated with water, it travels through an upper, "unsaturated" or "vadose" zone. In this area, the water moves between soil particles and rocks that contain both air and water. Some soil water will be taken up by plants; much of the rest will travel downward under the pull of gravity beyond the root zone of local vegetation and leaching into the "zone of saturation." At this point, the traveling water is considered "groundwater."

Think of a bowl filled with gravel or sand. Assume the fill material reaches the top of the bowl and think of pouring water to only partially fill the container. The line that corresponds to the top of the water level corresponds to what is known as the water table in the ground.

If the aquifer is unconfined -- at least in part -- water entering from the ground surface will recharge this zone of saturation. The water table line will fluctuate depending on how much water remains stored in the ground and how much and how quickly it is replenished. (See the figure below.) During periods of drought, in areas where pavement and other impermeable surfaces cover the ground or in areas where many pumping wells are pulling water out of the aquifer, the water table can drop. In times of heavier rainfall or diminished pumping, the water table may rise.

As a general but not ironclad rule, the topography of the water table will correspond roughly to the topography of the land, with the slope of the land surface frequently indicating the direction of groundwater moving with gravity. Generally, the groundwater flows from the areas where it enters the soil (the "recharge areas") "downhill" or "downgradient" toward "points of discharge" -- such as a spring, a river bed, a wetland or a pumping well.

water table diagram
Water situated in the water-saturated zone below the earth is called groundwater. The top of this zone (top of the darker area) is called the water table. The water table fluctuates over time and generally follows the topography of the land. Where it intersects rivers and wetlands, the groundwater often becomes surface water. Diagram adapted from USGS, Ground Water, 1993.

As noted earlier, the rate of groundwater movement can vary dramatically depending upon local conditions. While stream water may be moving at a rate of feet per second, groundwater generally moves much more slowly, sometimes only a few feet each month or year. Only in areas of limestone or "karst" topography, where solution channels form in the bedrock, will groundwater flow resemble that of surface water.

The extreme variation in rates of groundwater movement arises from a combination of several factors, including the type of water-bearing material, geographic features, and the rate and location of groundwater withdrawals by supply wells.

Wells can alter both the speed and direction of natural groundwater flow, and the influence created by them can dramatically affect the “natural” or undisturbed path of groundwater.

Pumping Wells Create a Cone of Depression in the Water Table

Adapted from Pennsylvania Groundwater Education Project, Groundwater: A Primer for Pennsylvanians, undated.

A pumping well pulls groundwater in the surrounding aquifer toward it, creating something of a "valley" within the aquifer. This lowering of the water table in a cone-like shape, with the well opening at the bottom center point, is called a "cone of depression."

Again, depending on a variety of localized conditions and pumping rates, this cone may be relatively small and fairly regular in diameter or it may extend long distances in an irregular pattern.

The closure of wells, likewise, can alter the predicted rate and course of groundwater movement -- and of contamination movement within the groundwater regime. In trying to better understand local contamination threats to underground water supplies, it is important to remember the dynamic nature of the resource. Programs that assume static conditions can easily misjudge how to manage or prevent contamination problems.

1. Alexander, Calvin E., Farming and Groundwater, Agricultural Law and Policy Institute,1988.