Groundwater is important globally for human consumption, and changes in quality can have serious consequences. It is also important to sustain habitats and to maintain the quality of base flow that feeds the rivers. The chemical composition of groundwater is a measure of its suitability as a source of water for human and animal consumption, irrigation, industry and for other purposes. Also influences the health and functioning of ecosystems, it is important to detect changes and give early warnings of changes in quality, both in natural systems such as those resulting from pollution.
Salinity: The fresh groundwater can be limited laterally (in coastal regions) for interfacing with the marine water and adjacent rock types, or vertically in the water from the underlying formations. The salt water intrusion into coastal aquifers may be the result of intensive pumping of fresh groundwater, or the result of the decrease in the flow of a stream (eg due to the construction of dams or diversions), which leads to reduce aquifer recharge in the alluvial plains and deltas. The intense evaporation in areas with low water table depth may also lead to salinization. Variations in salinity levels can occur due to natural climate change or over-pumping and irrigation practices that encourage the precipitation of dissolved solids such as salts on agricultural lands. It is important to monitor all changes in salinity using Cl (chloride) or SEC (electrical conductivity) and, if possible, characterize the source of salinity, using one or more of the secondary indicators.
Acidity and redox state (oxide reduction): Industrial emissions of SOx and NOx have led in some places, to reduce by an order of magnitude the average pH of rainfall. This has accelerated the rate of natural weathering and reduced the attenuation capacity of soils and rocks, causing an increase in the acidity of the shallow groundwater, especially in areas with a carbonate mineral. In large areas of North America, Northern Europe, Southeast Asia and South America, acidification is a major problem for human health and ecosystems. The impact on surface water is worse in places where the effects of decreased attenuation of HCO3 in the base flow of groundwater that feed rivers and lakes. Changes in the status of oxidation-reduction (Redox) of groundwater (mainly as a consequence of reduced O2) can also take place rapidly due to microbial or chemical processes in natural systems or the resulting pollution. Increased acidity (decreasing pH) or a decrease in Eh (redox potential) may lead to undesirable increases in dissolved metals. However, the onset of reducing conditions can lead to benefits such as de-nitrification in situ.
Radioactivity: The natural background radiation can be closely related to the presence or absence of sediment and rocks containing uranium or other radioactive materials naturally. Rn concentrations of dissolved gas is a way of detecting the presence of natural radioactivity in groundwater [see: karst activity]. From an environmental point of view is more meaningful to the possible migration of radionuclides from groundwater test thermonuclear, nuclear power plants or military installations.
Pollution of agricultural origin: During the last decades in most countries, nitrate levels in groundwater have been increasing as a result of drainage of excess fertilizers. Nitrate, and other parameters derived from fertilizers such as K (K / Na), DOC and SO4, serve as important indicators of environmental degradation caused by man, although denitrification can also occur under natural conditions of reduction (see : Acidity). Herbicides and pesticides (insecticides, fungicides) and other chemicals, can also be mobile in groundwater and serve as an index of pollution of agricultural land under for the past 20 to 30 years. Because the analysis is extremely difficult, it is not feasible to use them as indicators. However, its presence can be inferred if there is high concentrations of other indicators.
Pollution from mining: sulfate derived from oxidation of sulphide minerals is the best single indicator of pollution from the mining of metals and coal, the production of oil and gas and to a lesser extent, exploration activities . A decrease in pH is usually associated with this process, as well as increases in loads of dissolved Fe and other metals could contaminate both surface and groundwater in the form of acid mine drainage. The problem becomes critical for water supplies and ecosystems where the water table rise after the closure of a mine. The F and Sr derived from weathering of minerals associated with the grain can also serve as secondary indicators.
Pollution from urban and industrial use: The impact of human settlements and the accumulation of waste, characterized by numerous chemical compounds, it is always evident in the quality of local groundwater. Many chemical compounds into the ground, but the deterioration of water quality can be assessed through those constituents who are more mobile. A key aspect would be to protect the deeper aquifers are not contaminated, and monitor the effects of pollution plumes that move in the surrounding areas. Thus, DOC, Cl and HCO3 are primary indicators of pollution in towns, cities, waste dumps and landfills. The biological impacts can be measured using indicator organisms such as E. coli. However, harmful microorganisms usually fade gradually after traveling several hundred feet into the ground water, so an alternative is to measure the decomposition products of these biological processes, such as DOC and HCO3. The indicators include side B (which are used detergents), solvents and hydrocarbons.
HUMAN OR NATURAL CAUSE
Both. Changes in natural conditions could occur during the original time scale we are interested and can be measured in a single borehole or a spring. However, superimposed on these, are the major impacts of human activities described above.
ENVIRONMENT WHERE APPLICABLE
The main importance of environments, from a global viewpoint, are those where major aquifers are sources of supply of water, especially those at ravines with coastal or deltaic sediments saturated generally limited thickness and high transmissivity. These environments include temperate regions where there is no adequate supply of surface water, and semi-arid regions where groundwater is, inevitably, the only source of supply, and humid tropical regions, where groundwater means, increasingly, a source bacteriologically “safe” drinking water. In such cases it is essential to protect groundwater quality. Furthermore, in these regions, groundwater provides an important means to monitor environmental change partners.
Monitoring sites
Deep wells, springs and wells of water main where the flow is active. Avoid observation wells where the flow can be low or stagnant. Monitoring wells should be located along the main hydraulic gradients, and some should be located downstream of potentially problematic areas (eg power plants, urban areas, waste dumps, agricultural land) in order to relate the individual pollutants with their sources. Wherever possible, we must integrate the monitoring of groundwater geoindicadores with national water quality.
SCALE SPACE
Plot mesoscale / regional.
MEASUREMENT METHODS
The first-order indicators can be analyzed with relative ease using standard techniques and laboratory equipment. In many cases could be measured by remote sensors located in boreholes or at discharge. The measurement of small environmental changes requires high precision and accuracy. Changes in status should be assessed using acid HCO3 rather than pH, since the latter may vary little (except below a pH of 5.5) due to the effect of attenuation. Indicators of second-order analysis requires more specialized and expensive, such as measurements of radioactivity.
FREQUENCY OF MEASUREMENT
Changes in groundwater quality are usually seen with seasonal or annual scale. Dispersion processes, and reaction mixture involves the addition of small amounts of contaminants is commonly difficult to detect. Both the changes at the regional level as the effects of a point source are important to monitor. Suggests a maximum frequency of 4 times per year to detect changes in the water too shallow groundwater in annual change measurements are sufficient to deeper sources.
LIMITATIONS OF DATA AND MONITORING
It is necessary to take great care to ensure that the sampling sites are representative of the flow regime of groundwater, both vertically and horizontally. It is extremely useful to have two sampling points, one shallow and one deep, installed on the same site. The spatial variability imposed a limit of what can be achieved, and it is likely that the resulting network of sites for measuring the quality of groundwater represents a compromise. The accuracy between analytical measurements and far between, possibly made by different people, you will probably be a problem. The springs can be stable in the long term, but may fluctuate rapidly due to the dual nature of the porosity of the aquifer, changes in atmospheric pressure in the rates of precipitation and evaporation, or by volcanic or seismic activity, making difficult to determine the causes.
APPLICATIONS TO PAST AND THE FUTURE
Below the groundwater level, groundwater is usually not a good file, the changes in the past, due to the dispersion of the signal. However, in large sedimentary basins, the paleo-waters can be recognized by means of chemical and isotopic signals. Around springs, deposits of siliceous or calcareous material (travertine, tufa, tuff), ranging from inorganic precipitates which are those that are totally organic, may reflect past changes in climate conditions in surface water and groundwater chemical premises. The chemistry of groundwater in the unsaturated zone can provide a registration key to past climate changes and ecological [See: Chemistry of groundwater in the unsaturated zone].
POSSIBLE THRESHOLDS
The World Health Organization (WHO, 1993) and many agencies have established rules or standards for maximum allowable concentrations of various substances in drinking water. A variety of guidelines to establish the quality of groundwater used for other purposes, such as livestock and irrigation.
GEOLOGICAL AND ENVIRONMENTAL ISSUES RELATED
In many areas, especially in tropical regions, endemic human diseases (such as fluorosis associated with excess fluoride and goitre associated with iodine deficiency) are related to the natural quality of groundwater, these diseases may be mitigated by appropriate changes in water chemistry. Changes in groundwater quality could be, as detailed above, the result of other environmental impacts, including acid rain, urbanization, agricultural development, land clearing and mining. Variations in the chemistry of groundwater can also cause changes in habitats and causing the salinization of soils and surface waters. In several countries over the past 30 years, the tritium content of groundwater has been monitored with respect to the drop in waste from nuclear tests.
OVERALL ASSESSMENT
Monitoring changes in groundwater quality provides a key indication of both human impacts on the hydrosphere and in the general environment.
