What does carbonic acid in groundwater do

The buffering capacity of an aquifer is not made up solely of the groundwater alkalinity. Minerals can contribute substantial buffering capacity, particularly in aquifers that contain calcium carbonate the mineral calcite; CaCO 3. When acid enters an aquifer containing calcite, the calcite dissolves releasing carbonate ions. Conversely, influx of alkaline water can cause calcite to precipitate, removing carbonate ions from solution.

In response, some bicarbonate ions will deprotonate, buffering the pH against the increase. Aquifers in limestones or other rocks rich in calcite have higher buffering capacities and groundwater alkalinities than aquifers containing no calcite. Other minerals common to aquifers can also act like pH buffers.

Silicate minerals such as feldspars and clays will dissolve by consuming acid in acidic conditions. Under high pH conditions, they dissolve by consuming base. These minerals dissolve incongruently and their reactions are not reversible.

Therefore, they are not true pH buffers, but they do provide resistance to pH change in groundwater. However, silicate mineral reactions are generally slow compared to carbonate minerals and dissolution of most native silicate minerals will not be sufficient to counteract pH change caused by the influx of an acidic contamination plume or vigorous microbial processes that produce acid.

Surfaces of clay minerals and similarly-sized oxides in aquifers can also act as pH buffers. The surfaces of these minerals can reversibly bind and release hydrogen ions as pH changes Fig. Broken bonds at mineral surfaces leave oxygens that are not fully bonded and act as weak acid anions that adsorb hydrogen ions and other cations.

Influxing acid can be neutralized by adsorbing hydrogen ions on negatively charged surface sites and influxing base can be neutralized by adsorbing to positively charged sites.

The zero point of charge is the pH value at which the net surface charge density is zero. The buffering capacity for acid or base depends on how far the pH of the system is above or below the zero point of charge. The point of zero charge is different for different minerals and lists are available from many sources [2]. The total aquifer buffering capacity is the sum of the buffering capacity of the groundwater plus that of the aquifer minerals and is determined by acid-base titration.

A variety of methods exist for measuring buffering capacity examples [7] [8] , or Standard Method [9] can be modified for solids , but the method should be consistent with the end-use of the measurement. The slow kinetics of some pH buffering reactions means that various methods may yield different results depending upon equilibration time. There are many cases where raising and buffering pH are beneficial to the environmental remediation process.

Anoxic limestone drains have been used to raise the pH of acid mine drainage by reacting the acidic metal contaminated water with calcite, promoting precipitation of the contaminant metals. Likewise, groundwater contaminated by metals and radionuclides is often acidic and raising pH to natural values can increase in situ adsorption and precipitation of the contaminants [10].

Iron can be removed by oxidation, sedimentation, and fine filtration, or by precipitation during removal of hardness by ion exchange not a recommended practice. Sources of nitrate NO 3 - are decaying organic matter, legume plants, sewage, nitrate fertilizers, and nitrates in soil. Nitrate encourages growth of algae and other organisms that cause undesirable tastes and odors. Concentrations much greater than the local average may suggest pollution.

Nitrate in water may indicate sewage or other organic matter. In amounts less than 5 ppm, nitrate has no effect on the value of water for ordinary uses.

Chiefly, "dissolved solids" is the total quality of mineral constituents dissolved from rocks and soils, including any organic matter and some water of crystallization. Water containing more than 1, ppm of dissolved solids is unsuitable for many purposes. The amount and character of dissolved solids depend on the solubility and type of rocks with which the water has been in contact. The taste of the water often is affected by the amount of dissolved solids.

In most water, nearly all the hardness is due to calcium and magnesium carbonates. All of the metallic cations other than the alkali metals deposit soap curd on bathtubs. Hard water forms scale in boilers, water heaters, and pipes. Hardness equivalent to the bicarbonate and carbonate is called carbonate or "temporary" hardness because it can be removed by boiling.

Any hardness in excess of this is called noncarbonate or "permanent" hardness. Noncarbonate hardness is caused by the combination of calcium and magnesium with sulfate, chloride, and nitrate.

Scale caused by carbonate hardness usually is porous and easily removed, but that caused by noncarbonate hardness is hard and difficult to remove. Hardness is usually recognized in water by the increased quantity of soap or detergent required to make a permanent lather.

As hardness increases, soap consumption rises sharply, and an objectionable curd is formed. In the development of a water supply, hardness is one of the most important factors to be considered. In general, water of hardness up to 60 ppm is considered soft; 61 to ppm moderately hard, to ppm hard, and more than ppm very hard. Water turbidity is attributable to suspended matter such as clay, silt, fine fragments of organic matter, and similar material.

It shows up as a cloudy effect in water and for this reason alone is objectionable in domestic and many industrial water supplies. Filtered water is free from noticeable turbidity.

Unfiltered supplies, including those that contain enough iron for appreciable precipitation on exposure to air, may show turbidity. In surface water supplies, turbidity is usually a more variable quantity than dissolved solids. Color refers to the appearance of water that is free of suspended matter. It results almost entirely from extraction of coloring matter and decaying organic materials such as roots and leaves in bodies of surface water or in the ground.

Natural color of 10 units or less usually goes unnoticed and even in larger amounts is harmless in drinking water. Color is objectionable in the use of water for many industrial purposes, however. It may be removed from water by coagulation, sedimentation, and activated carbon filtration. However, it may cause mottling of the teeth depending on the concentration of fluoride, the age of the child, the amount of drinking water consumed, and the susceptibility of the individual.

A small number of minerals comprise nearly the entire mass of sandstone aquifers. The average sandstone, as determined by F. They also identified four geochemical markers to help monitor sites and discover when CO2 has leaked and caused metals to move into the groundwater. Scientists have already conducted short-term experiments of two-weeks to one month and found that CO2 in very small amounts can escape along rock faults and old petroleum wells into near-by groundwater and release harmful metals such as arsenic and uranium into the water.

Once CO2 reaches a freshwater aquifer, the quality of the drinking water is site specific, and depends on an array of factors including the size of the leak and the types of bacteria in the water, Little said.

Other researchers are trying to determine how a very large leak might affect the subsurface environment, while the Department of Energy DOE and private investors are beginning studies of potential groundwater contamination in the field, rather than in a lab as Jackson and Little did. The paper was published just as EPA finished a rule designed to protect potential drinking water sources from contamination following a CO2 leak.

Announced on November 22, the rule is written for the owners and operators of potential CCS wells. Rather, the rule leaves many of the decisions about site selection and permit approval up to each state. There is a definite shift in certain parts of country to use saline or more brackish water for drinking.

Groundwater protections should be in place for areas in the southwest, such as Las Vegas, where utilities are having a difficult time finding water sources, Lane said. After observing the CO2 percolating through aquifer sand and sediment for a year, Jackson said the study strongly suggests to him that long-term monitoring for CO2 leakage into freshwater aquifers should be part of every CCS project. The increased acidity caused by CO2 dissolved in water underground can cause metals to leach out of surrounding sand and rock.

Borrowed from agencies such as the US Geological Survey, the sediment used in the study was from 17 locations within four project sites: Acquia and Virginia Beach in the Virginia and Maryland tidewater region; Mahoment in Illinois; and Ogallala in the southern high plains of Texas.

The scientists dried the sediment samples and placed them in bottles, then piped a stream of Jackson and Little used their observations of the leaking CO2 to develop selection criteria, based on the metal contamination seen in the water, to help owners and operators choose CCS sites that are less likely to contaminate nearby freshwater aquifers. They also identified four geochemical markers to help monitor sites and discover when CO2 has leaked and caused metals to move into the groundwater.

Scientists have already conducted short-term experiments of two-weeks to one month and found that CO2 in very small amounts can escape along rock faults and old petroleum wells into near-by groundwater and release harmful metals such as arsenic and uranium into the water.

Once CO2 reaches a freshwater aquifer, the quality of the drinking water is site specific, and depends on an array of factors including the size of the leak and the types of bacteria in the water, Little said.