Water Well Journal
Issue link: http://read.dmtmag.com/i/617280
Samples from the same well collected several hours apart exhibit- ing different discoloration and variations in water chemistry and microbiology. Oftentimes, it is useful to actively pump (purge) the well before collecting a sample. Photo courtesy Edd Schofield, Johnson Screens. pair with carbonate or non-carbonate ions to form mineral precipitates. Most of the time, hardness consists of cal- cium (Ca), magnesium (Mg), and iron (Fe), three of the more common ions in groundwater. Usually, 99% of hardness readings are made up of the calcium and magnesium concentrations. Hardness levels of 0-60 mg/L are classified as soft; levels from 61 to 120 mg/L are moderately hard; levels of 120 to 180 mg/L are hard. Water with a hardness level greater than 180 mg/L is classified as very hard. Hardness is generally reflective of the supporting aquifer. For example, the carbonate-rich Ogallala Aquifer exhibits very high levels of carbonate hardness, often reaching 300 to 400 mg/L. Any increases in hardness, either steady or sudden, may reflect the concentration of mineral-forming ions within the well. Iron This element is a common parameter of concern for groundwater and is eval- uated in several methods. The dissolved iron test analyzes for ferrous iron (Fe +2 ), which is iron in solution in the first stage of oxidation, usually as ferrous oxide (FeO) or fer- rous sulfide (FeS). Ferrous oxide can represent iron just released from a sur- face, such as a well casing sidewall, as a result of oxidation, which can indicate corrosion. Iron in this ferrous state may also be reflective of native background iron within the aquifer; however, it is usually further oxidized immediately as it enters the borehole to ferric iron (Fe +3 ). Therefore, determining the fer- rous iron (Fe +2 ) presence in a water sample can indicate active corrosion that is occurring within a system. The level of dissolved iron can indicate the severity of the corrosion. Typically, groundwater wells exhibit dissolved iron (Fe +2 ) levels below the detection level of 0.02 mg/L. The total iron test is reported as iron (Fe) and represents for both ferrous iron (Fe +2 ) and ferric iron (Fe +3 ) for a meas- ure of the total iron in a sample. Ferric iron (Fe +3 ) is iron that has been further oxidized, moving from ferrous oxide (FeO) as ferrous iron (Fe +2 ) to ferric iron (Fe +3 ). Total iron levels above 1.0 mg/L in groundwater wells can be an indicator that iron precipitation is occurring. Increases in total iron can indicate problems are occurring within the well and aquifer. Iron bacteria, corrosion, aeration, and declining water levels can all cause increases in the iron level. Like several of these parameters, iron levels can fluctuate with well activity, and as such, the well's use or inactivity should be considered when evaluating results. Microbial Presence Bacteria are universally present in our groundwater aquifers. The popula- tion size and type are generally used to evaluate problems associated with bac- teria such as biofouling, foul odors, and the presence of pathogens. While the total coliform test is commonly per- formed, it does little to tell us of the res- ident bacteria nor does it alert us to the thousands of other microbial species that can inhabit a well. Important microbiological parame- ters for well evaluation include an assessment of the total population, the presence of specific troublesome organ- SCHNIEDERS from page 23 24 January 2016 WWJ waterwelljournal.com