Human Influences on Groundwater Availability
Prior to a recent knowledge about groundwater resources, it was commonly thought that groundwater was so isolated from the land surface that it was unaffected by human activities. As a more complete scientific understanding of groundwater developed, it became clear that large-scale agricultural practices and expanded urban centers created groundwater issues. If more people had known some basic facts about groundwater in the 1930’s through the 1960’s, many of the legacy water contamination and water depletion issues could have been avoided.
Figure 1 – Estimated water use in the United States 2015. (Source: US Geological Survey, Circular 1441, 2018; Figure 15; https://pubs.usgs.gov/circ/1441/circ1441.pdf, accessed 2/27/20)
Groundwater Withdrawal for Human Use
Total water use in the United States almost doubled between 1950 and 1980 when it peaked and has tapered off measurably between 2005 and 2015 (Figure 1). Groundwater withdrawals were less than surface water but mirrored the increase in use until 1980 at which time the amount of groundwater withdrawn remained nearly constant. Because groundwater now represents a larger percentage of total water used, we are now more dependent on it than ever before. The reliance on groundwater for human use has placed new stresses on the groundwater system and has caused water levels to drop where pumping has increased.
Point and Nonpoint Sources of Contaminants
The more groundwater withdrawn also increases the likelihood of contaminants entering aquifers which can be contaminated by point sources and nonpoint sources. Examples of point sources include leakage from gasoline storage tanks and seepage from landfills. Nonpoint sources of contaminants cover larger areas and, usually, at lower concentrations than point sources. For example, agricultural fields, in aggregate, represent large areas through which fertilizers and pesticides can move into the groundwater system.
Agricultural Development
Irrigation
Between 1900 and World War II, groundwater was not a major source for irrigation and the pumps to lift groundwater from depth had not been invented (Figure 2). After World War II, the more common use of submersible pumps allowed water to be efficiently extracted from deeper wells which paved the way to use groundwater for irrigation more economically especially with the center-pivot water systems that came into use in the 1950’s when the midwestern U.S. was undergoing a period of extensive drought. This new technology was part of the trend toward larger, more mechanized farms. It also allowed many more private wells to be drilled for domestic water use. As a result, groundwater withdrawals rose from 34,000 million gallons per day (mgal/d) in 1950 to 83,000 mgal/d in 1975. Withdrawals have remained at about this rate ever since. Worldwide, the trend of groundwater use has generally tracked the usage trends in the U.S. although at generally lower volumes.
Figure 2 – Well pump that was typically used from the early 1900’s until the 1940’s. Pumps such as this could not lift water from deep wells, nor could they pump large quantities of water. This pump was initially powered by a gasoline engine and, later, after rural electrification, by an electric motor. (Photo from N.G. Grannemann)
Drain Tiles
Another trend in water-related agricultural practices involves the installation of tile drains to remove excess water from farm fields – the opposite problem that is addressed by irrigation. Tile drains are primarily installed in fields with poorly drained soils or in wetland areas that have been converted to farm fields. The drains remove excess water near the root zone and transfer it to surface water. This allows planting of crops that cannot grow in wet conditions. The rapid transfer of ponded water in fields to local streams disrupts the local water balance and may reduce recharge to the groundwater system. Recent developments in drainage technology have allowed farmers to drain their fields during wet periods (primarily in spring) and flood the drain tiles during dry periods for irrigation.
No-Till or Minimal Tillage
Another modern agricultural practice that impacts groundwater is directly planting seeds into soil that has not been broken by tillage equipment. The main purpose of this technique is to reduce soil erosion because small soil particles are not exposed at the land surface. No-till agriculture results in less soil compaction, more soil moisture, lower soil temperatures, and less transpired water all of which promote a more natural balance in recharge to groundwater. However, without tillage to kill unwanted weeds, herbicides are used in place of physically uprooting weeds. If applied correctly, little of these chemicals should infiltrate to the water table. One of the most used herbicides is Glyphosate. It readily adheres to soil particles, a quality that makes it less likely to be found in groundwater. Atrazine is another weed killer that is commonly used for row crops. It does not readily adhere to soil particles and is the most common herbicide found in groundwater in areas that are intensively farmed.
Nutrients and Insecticides
Nutrients often have elevated concentrations in groundwater in places with intensive farming especially in areas with sandy soils. The application and storage of fertilizer and animal manures can be sources of groundwater nutrient pollution. In the U.S., 16 states have more than half of their land use classified as farmland and 5 states have 90 percent or greater (Nebraska has 93 percent). With such vast areas being farmed, agriculture is a significant potential source of nutrients, especially nitrate, entering groundwater.
Insecticides, in contrast to herbicides, are not as commonly detected in groundwater and most of those detected are compounds that are no longer in common use. Although they occur infrequently, because of their toxicity, there is still concern about their occurrence, especially in water from wells used for drinking water.
Urban Development
Population shifts from rural to urban areas have been underway for many years on a worldwide scale. This shift has created much higher demand for water in urbanized areas. Most cities use surface water as the source of public supply. The groundwater beneath urban centers, however, is impacted by construction, underground infrastructure, and surface runoff. In addition, people who live in the suburban ring around most cities often rely on groundwater for supply. In these areas, some groundwater issues reflect both a rural legacy and a new urban demand for water.
Urban Infrastructure and Groundwater
Because the urban built environment contains a high proportion of impervious surfaces (buildings and pavement), it seems intuitive that groundwater recharge would be reduced. While surface runoff is generally higher because of these surfaces, what is often not considered is how much water is added to the groundwater system in urban areas. The sources of this recharge include leaking sewers and water mains, excessive irrigation of lawns and gardens, infiltration of runoff, and infiltration from septic systems (mostly a suburban issue). Because the soil and rock beneath cities has been extensively disturbed by underground infrastructure, new pathways for water to infiltrate to the water table have been opened. The amount of extra water in the subsurface is hard to quantify, however, in some older cities, the loss of water from pressurized water mains can be as much as 20 percent of total flow in the system. In some cities, this excess water has lead to rising water tables that create engineering issues including basement dewatering and weakened foundations.
Groundwater Contaminants in Urban Areas
Most urban areas use large volumes of polluting chemicals, a portion of which, inevitably, contaminates urban groundwater. Some are point sources such as leaking gasoline tanks, septic fields, or landfills. Others are nonpoint sources such as lawn fertilizer or leaking sewer pipes. The range and quantity can vary greatly from city to city.
Many urban areas in temperate climates use road salt to maintain vehicle safety in the winter. The infiltration of this salt into groundwater is a common issue in temperate climates. Small to medium sized cities may use groundwater as a source of drinking water and salty water causes human health concerns. Cities that do not use groundwater as a source have greater concerns with ecosystem effects of urban contamination. Pumping for supply or dewatering can cause the distribution of contaminants in groundwater to move from source to well.
Discussion about Urban Development and Groundwater
After reading the informational part of this lesson, comment on the water cycle depictions in Figures 3 and 4. Are the hydrologic and contaminant impacts of urban areas on groundwater correctly depicted? Is the term “High groundwater flow referring to volume or velocity or both? How will the elevation of the water table change due to urban development?
Figure 3 – The natural water cycle (Source: Auckland Council, 2010; https://www.greaterauckland.org.nz/2017/12/15/aucklands-urban-freshwater-historic-relationship/water-cycle/ accessed 2/27/20)
Figure 4 – The urban water cycle (Source: Auckland Council, 2010; https://www.greaterauckland.org.nz/2017/12/15/aucklands-urban-freshwater-historic-relationship/water-cycle/ accessed 2/27/20)