By Darrel Dunn, Ph.D., PG, Hydrogeologist
The purpose of this web page is to present my comments on groundwater topics addressed in emails from viewers of this site. These comments are brief statements related to questions and comments in the viewer's emails. They are not comprehensive coverage of the topic.
Gas in Groundwater
Methane and hydrogen sulfide are common dissolved gasses in groundwater. Methane can produce an explosive atmosphere if concentrated in an enclosed space in a house or other enclosure. The web pages on this site titled Subsurface Methane West US and Subsurface Methane New Mexico contain much information on the occurrence of methane in groundwater.
Low concentrations of hydrogen sulfide may impart an unpleasant odor often described as a "rotten egg" smell. Hydrogen sulfide may be produced by reduction of sulfate in groundwater that has a low oxygen concentration. Such chemical reduction may be augmented by the action of anaerobic bacteria if organic material is present. Hydrogen sulfide in the low concentrations associated with the foul odor may cause health effects. An OSHA Fact Sheet says that "repeated or prolonged exposures may cause eye inflammation, headache, fatigue, irritability, insomnia, digestive disturbances and weight loss." A CDC Fact Sheet says that hydrogen sulfide has not been shown to cause cancer in humans. This fact sheet also states that the EPA has determined that data for hydrogen sulfide are inadequate for carcinogenic assessments. Hydrogen sulfide is corrosive to metals, can cause yellow or black stains on kitchen and bathroom fixtures, and can discolor and affect taste of beverages and cooked foods.
Two ways to deal with gas in water from a well are (1) explore for productive layers in the aquifer that do not contain gas, and (2) remove the gas from the water produced by the existing well. One way to remove gas from water is by aeration. There are various ways to aerate the water. Some are installed within the well, others treat the produced water.
Acrylamide in Groundwater
Operations that clean or process sand from sand mines commonly use polyacrylamide (PAM) as a flocculant to remove unwanted minerals and fines from water used to wash the sand. Acrylamide (AMD) is a chemical used in the production of polyacrylamide. Therefore, residual acrylamide may be present in industrial sand wash water, wastewater, and sludges. Acrylamide is water soluble and unlikely to adsorb to organic and inorganic soil components. Consequently, potential for groundwater contamination may be a risk associated with the use of PAM as a flocculant. Polyacrylamide itself is generally considered not toxic, but acrylamide is considered by the United States EPA to be a likely human carcinogen and neurotoxin.
Groundwater Age Dating
The age of groundwater ranges from less than a month to more than a million years. The following image shows some of the methods for dating the age of groundwater and the age for which each method is applicable.
Argon (Ar) is a dissolved gas, so sample collection without dissolution of the gas involves special equipment used with great care. The helium (He) method also requires dealing with a dissolved gas, plus the interpretation of the laboratory results is complicated. Sample collection for Carbon-14 is about as easy as filling a bottle. However, analysis of laboratory results is complicated by chemical reaction between carbon in the groundwater and carbon in such rocks as limestone. Post-processing of laboratory results to adjust for the carbon reactions increases uncertainty.
Interpretation of laboratory results for groundwater age is very complex. One problem is that almost all groundwater samples are mixtures of water of varying age, combining all of the flowlines reaching a well or spring. Another problem is that the groundwater may contain a very small proportion of connate water diffused from clay or other low permeable material. If the connate water is extremely old even a very low admixture can cause the water to appear to be much older than it really is. There are many other problems.
Another way to estimate the age of groundwater is to develop a computer model of the system and use particle tracking. It might be useful to compare particle tracking results to the results of dating the age of a water sample.
Coastal Saltwater Intrusion
Real groundwater systems affected by saltwater intrusion are dynamic and the Ghyben-Hertzberg relation, being based on a static system, does not strictly apply. Both the saltwater and the fresh water are moving and the movement changes with time. The systems have a history, and at any given time you are just seeing the state of the system at that time. Ghyben-Hertzberg helps one understand the saltwater-freshwater relationship, but the principles of geology, chemistry and physics determine it. If the characterization of the system is over-simplified, it may be wrong. More information on saltwater intrusion may be viewed on the web page titled Saltwater Intrusion.
Groundwater in the Mancos Shale, Pagosa Springs, Colorado
The Cretaceous Mancos Shale is at the land surface in a large area extending northwest and southeast of Pagosa Springs in southwestern Colorado. The area is on the northeastern flank of the Archuleta Anticlinorium. The anticlinorium is on the northeastern side of the San Juan Basin. The Dakota Sandstone, which underlies the Mancos Shale is exposed along the crest of the anticlinorium. The eroded Mancos Shale is over 1000 feet thick thick in the eastern part of the area and thins southwestward toward the exposed Dakota Sandstone.
Water wells in the Mancos Shale are as much as 200 feet deep and yield from 1 to 30 gallons per minute (gpm) . The water is produced from fractures and discontinuous sandy layers. The quality of the water from the Mancos Shale is generally poor. Total dissolved solids (TDS) range from 1000 to 2500 milligrams per liter (mg/l), with high concentration of sodium, calcium, iron, and sulfate. Dissolved hydrogen sulfide is common.
Where the Mancos Shale is thin or absent, wells are completed in the Dakota Sandstone. The water comes from fractures. Yields range up to 30 gpm. The quality of the water is variable but some TDS is less than 1000 mg/l.
Houses in areas where the high water table elevation is less than one foot below a basement floor generally need a sump pump installed. The water table is roughly the top of the saturated portion of the soil beneath the house. It generally fluctuates up and down seasonally due to percolation of rain and snowmelt. It also has long term trends related to drought and rainy periods. Human activities can also affect the rise and fall of the water table (such as lawn irrigation). The purpose of the sump pump is to discharge enough water to keep the water table from rising to the lowest floor or the foundation footer of the house, thus preventing damage.
Mining hydrogeology is the application of hydrogeology to the successive stages of mining projects. These stages may include:
Scoping feasibility study,
Preliminary feasibility study,
Detailed feasibility study,
During the feasibility stages, hydrogeologic studies should be performed if the cost of dealing with water issues is likely to be significant during the permitting and later stages. These issues may include:
Mining impacts on water quality,
Mining impacts on water wells,
Mining impacts on streamflow,
Mine dewatering requirements,
Acid mine drainage,
Ground subsidence effects.
Adequate feasibility assessment can be very important, because a misleading assessment can result in rejection of a viable mining project or, conversely, further expenditure on a non-viable project.
During the scoping feasibility study, the hydrogeologic assessment of requirements for dealing with water issues may be based on pre-existing data and comparison with similar existing mines, if any. For dewatering assessment, a "large well" analog might be used along with hydraulic conductivities derived from existing well specific capacity data.
If the scoping feasibility study does not eliminate the project, the ensuing preliminary feasibility study may be based on recommendations for data collection and analysis in the scoping study. Some hydrological data may be collected in conjunction with early geological exploratory work. More realistic hydrologic modeling might be used, such as analytic element modeling and analytic stream depletion calculations.
If the preliminary feasibility study does not eliminate the project, a detailed feasibility will include any hydrologic data collection recommended in the preliminary feasibility study, such as:
Pumping or injecting testing in geology exploration holes to acquire data on hydraulic conductivity and other hydraulic properties, especially the effects of fracturing,
Testing wells constructed specifically to fill in critical groundwater data.
The detailed feasibility study may include numerical groundwater flow modeling to evaluate dewatering scenarios and surface water depletion.
If the detailed feasibility study does not eliminate the project, pre-mining hydrogeologic activities may include:
Design and installation of groundwater monitoring systems,
Pre-mining water quality data collection and assessment,
Pre-mining monitoring of groundwater levels,
Pre-mining monitoring of surface water flow,
Establishment of weather stations to help separate hydrologic effects of weather from the effects of mining,
Identifying and quantifying water supply sources,
Designing water control systems including dewatering alternatives that minimize cost and provide depressurization,
Permit application support including an environmental impact assessment.
During the mine construction and the extraction phase, hydrogeologic activity may include:
Updating groundwater modeling to track the hydrologic effects of the mine and modify dewatering operations,
Implementing in-mine drainage boreholes to adapt to fractures, faults, and permeable rock encountered,
Surface water monitoring.
Mine closure may include:
Modeling the effects of mine closure on the groundwater system, water wells, surface water flow, groundwater quality, and surface water quality,
A scientific paper published by United States Geological Survey hydrologists Ward Sanford and David Selnick titled Estimation of Evapotranspiration Across the Conterminous United States Using a Regression with Climate and Land-Cover Data uses a regression equation to estimate fraction of precipitation lost to evapotranspiration. They present a map that shows an estimated value for every county in the conterminous United States. This map can be used with local precipitation data to estimate groundwater recharge due to seepage to the water table of precipitation that is not returned to the atmosphere as by evaporation and plant transpiration. The map is reproduced below.
Rcharge estimates may be made by various other methods. One method is Blaney-Criddle. The Blaney-Criddle method uses the following equation:
u is monthly evapotranspiration (consumptive use),
k is a crop coefficient, and
f = (t X p)/100
where t is mean monthly temperature in degrees Fahrenheit, and
p is monthly percentage of daytime hours of the year.
The coefficient k has been empirically determined for many "crops" in many geographic locations. Crop coefficients have been derived for natural vegetation as well as agricultural crops. The literature on Blaney-Criddle crop coefficients is extensive.