Hydrogeologic issues revolve around water quantity, quality and sustainability. Understanding climate uncertainties and developing better climate models is becoming more important to the creation of prudent water-resource strategies, with more emphasis placed on the disparities between rural and urban areas.
We are re-evaluating the need to understand past and future groundwater use and hydrogeologic conditions as they relate to the development of seismic hazards zones. When water-saturated sandy soils are subjected to ground shaking from earthquakes, the soil liquefies, loses strength and behaves as a viscous liquid, much like quicksand.
In areas of limited subsurface data, the California Geological Survey is taking another look at certain geologic criteria to assess whether such risks exist. Some of these areas are characterized by soil deposits of late Holocene age, where the anticipated depth of saturated soil is less than 20, 30 or 40 feet, depending on whether the estimated probable peak acceleration was exceeded over a 50-year period. In some closed basin environments, the historic high-water level is typically defined as the depth to saturated soil. The depth used to be less than 50 feet in some areas, but because of groundwater extraction over the decades, current depth levels are now greater than 100 feet.
Although these levels may never rise enough to be of concern in an earthquake, artificial recharge or water banking may lead to significant rises in groundwater levels over a 50-year period. These issues and the classification of an area within a seismic-hazard zone can have considerable impact on land-use designation, insurance and other development-related issues.
In the U.S. Southwest, the need to augment existing water supplies via desalination is becoming more attractive. Overall desalination costs continue to decrease as compared to other conventional water-treatment alternatives and have become more appealing, given the cost of importing water. Although disposal of the highly saline waste stream is not an issue near the Pacific coast, disposing the waste stream at inland sites remains problematic. The U.S. Bureau of Reclamation and communities in the Southwest are addressing this challenge.
In the environmental field, hydrogeologists are focusing more on contamination pathways such as exposure pathways into buildings and vapor exposure through indoor air than in the past. While some investigators are using the Johnson and Ettinger model, others are using tables supplied by regulatory agencies to define water-quality goals based on exposure pathways. Vapor surveys and profiles have become increasingly popular methods to study migration upwards into buildings. MTBE, perchlorate and heavy metals remain pertinent issues in some areas. Dry-cleaner compounds are a bigger problem, as PCE in sewer lines affects more water supplies. The issue is a public-policy concern because MTBE is difficult to remove from groundwater. Tert-Butyl alcohol, often associated with MTBE, and perchlorate, a strong oxidizer used in rocket fuels, air bags and road flares, have also become important contaminants to track. Perchlorate has been detected in groundwater in more than 20 states.
Mine pit lakes are still a challenge. About 80 existing pit lakes and 35 proposed pit lakes are scattered throughout the West. These pits can be quite large; some in Nevada, for example, may contain up to 540,000 acre-feet of water. Groundwater models suggest that they derive groundwater from all sides of the pit; through evaporation, solutes concentrate and adversely affect overall groundwater quality of the pit lake and possibly the surrounding hydraulically connected aquifers. The end result may be water with relatively high concentrations of metals and other inorganic constituents that exceed promulgated standards, a condition possibly exacerbated by the fact that these pits are likely to remain for decades to thousands of years.
Many papers and seminars have focused on applying bioremediation and low-temperature geochemistry research to understanding variations of groundwater quality. When biological activity changes the pH or oxidation potential of groundwater, for example, the results are precipitation or solubilization of metals compounds. Classic cases are where metals such as chromium or thorium appear in monitoring wells near landfills or in other sources of soluble organic carbon or methane gas. In these situations, indigenous microbes have used alternative electron donors (such as sulfates and hexavalent chromium) for their respiration. When these compounds are reduced or oxidized, their solubility changes, leading to variant minerals or changes in equilibrium relations and a change in overall water quality.
Based on these observations, the remediation community has begun to "drive" the local groundwater setting in the geochemical direction necessary to accomplish the task at hand. The goal is to integrate hydrology, chemistry and microbiology research into practical applications. Over the next few years, divisions between these separate fields will become less evident.
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