Over-Pumping Groundwater and Arsenic

By Judith Schumacher-Jennings

Over-pumping groundwater in California increases arsenic levels in well water. Groundwater is increasingly used to supplement finite surface water supplies.  Aquifer levels are decreasing due to increased demand and decreased precipitation. Groundwater is one of the world’s most important resources.  Groundwater provides about half of all drinking water globally, including the United States.

The central valley of California accounts for roughly 20 percent of groundwater withdrawals in the United States. The central valley is an arid region that supports a $17 billion agricultural industry. In the region of the valley known as the San Joaquin Valley, groundwater is the main source of drinking water for about one million people.  High water demands stress aquifers, especially during extended droughts.

Arsenic occurs naturally

Arsenic is a ubiquitous, naturally occurring contaminant.  It is present in drinking water of many aquifers. When present in significant amounts, arsenic increases the risk of cancer, heart disease and diabetes.  Arsenic is transported to the San Joaquin Valley from the Sierra Nevada and coastal mountain ranges for millions of years by rivers cutting through arsenic-bearing formations. Clays at or near the surface are the primary host of transported arsenic. Arsenic absorbs onto clay surfaces in significant amounts in the San Joaquin Valley.

As the clays are buried over time, their increasingly restricted oxygen supply reduces arsenic in the clay at depths greater than 200 feet.  The arsenic dissolves into the water in the pores of the clay. Higher levels of arsenic in the aquifer result from anaerobic conditions where oxygen is lacking and arsenic becomes more soluble. These anaerobic conditions occur naturally in thick clay, in manganese and at lower elevations.

California aquifers

In the aquifers of the San Joaquin Valley, the greatest depth typically drilled for groundwater pumping is 1,640 feet. An aquifer consists of alternating layers of sand, gravel and clay. In California, the aquifer system consists of an upper aquifer, a thick clay confining unit known as the Corcoran clay and a lower aquifer. The upper and lower aquifers contain sands and gravels, as well as numerous thin clay layers.

When undisturbed, groundwater within the aquifer primarily flows horizontally through the sediments with highest permeability, typically sands and gravels. Initially, pumped groundwater comes mostly from sands and gravels, which have lower arsenic concentrations.

Arsenic within pumped groundwater of the San Joaquin Valley has been noted for decades. Approximately 10 percent of the wells tested within the last 10 years have shown arsenic. Maintaining water quality is vitally important as groundwater pumping increases to meet agricultural and domestic needs.

Worker adjusting pump gauge
Droughts stress aquifer use

Two long droughts, from 1986 to 1993 and 2007-2015, recently hit the San Joaquin Valley.  During both, over-pumping stressed the aquifer system. The over-pumping sucked larger volumes of water into the aquifer from less-permeable anaerobic clays, inducing the release of pore water with high arsenic concentrations. Groundwater pumping in the San Joaquin Valley has caused declines of about 200 feet in groundwater levels over the past century, leading to subsidence, or sinking of the land, as much as 30 feet from 1925 to 1970 or about eight inches per year.

In addition to groundwater depletion, over-pumping results in land subsidence and increased extraction of pore water from clay layers. Clay drainage causes most aquifer compaction and subsidence of the overlying ground surface. There is thus a link between land subsidence and groundwater arsenic concentrations. Historic subsidence highly impacted historic arsenic concentrations, but has virtually no impact on recent arsenic concentrations.  Arsenic levels slowly return to their original levels after the groundwater pumping decreases. This implies arsenic stops leaking from the aquifer over time. Thus, avoiding over-pumping of aquifers should gradually improve water quality for the San Joaquin Valley.

Reducing groundwater pumping to sustainable levels should decrease both the rate of subsidence and arsenic concentrations.  The aquifers will eventually recover to normal levels of arsenic. With a global trend toward increased use of groundwater, effectively managing water quality along with water quantity is essential to preserve the continued use of this critical resource.

Courtesy of Rossmoor News, August 8, 2018 edition.  Email Judith Schumacher-Jennings at sjmadrone@sonic.net

Source information for this article appeared in the June edition of peer reviewed journal Nature Communications by Ryan Smith, Rosemary Knight and Scott Fendorf.

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