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Scientific Investigations Report 2007–5075

Scientific Investigations Report 2007–5075

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Ground-Water Quality

Concentrations of nitrate and selected other constituents in ground-water samples collected in June and August 2006 are shown in table 1. Only five of the nine drive-point wells installed in the former sewage lagoon were capable of producing sufficient volume of ground water for sampling. As previously described, the well at site LSU2 was too silty to provide useful water-level measurements or collection of ground-water samples. Wells at sites LSC3 and LSC2 were initially incapable of producing more than 2 milliliters of water per minute, so the well points were extracted following the August 10 water-level measurement and a 6-in. length of stainless steel screen attached to 3/8-in. polyethylene tubing was installed using a retractable-open casing with a removable drive point. The stainless steel screens provided a greater area of well opening for flow of ground water into the well, but sampling was still unsuccessful. Fragments of peat material were removed from the well at site LSC3 when the stainless steel well screen was extracted, and additional fragments of peat were extracted from the well at site LSC4 during subsequent probing. Hydraulic characteristics of peat include low permeability and high capacity for water retention (Price, 1997). The occurrence of peat in the areas near the well screen of wells at sites LSC3 and LSC4 was consistent with the observed drainage of the water during well development and the subsequent lack of yield when the well was pumped. Although the number of wells sampled was less than planned, the variability in the measured concentrations was small suggesting these samples generally are representative of ground water beneath the former lagoon area.

Concentrations of nitrogen species in ground water beneath the fomer lagoon were small with respect to the U.S. Environmental Protection Agency’s (USEPA) maximum contaminant level (MCL) of 10 mg/L for nitrate in ground water. Concentrations of nitrate measured in ground-water samples were less than the laboratory reporting limit of 0.06 mg/L, although other forms of nitrogen more compatible with reducing geochemical conditions that were present in the ground water were measured. Concentrations of total nitrogen, which is the sum of nitrate, nitrite, ammonia, and organic nitrogen, ranged from 1.15 to 8.44 mg/L as N. Most of the total nitrogen present in the ground-water samples was in the ammonia form, typically comprising 70 to 90 percent of the total nitrogen. Organic nitrogen was not measured directly but was calculated as the difference between the concentration of total nitrogen minus the inorganic forms nitrate, nitrite, and ammonia. If all forms of nitrogen were converted to nitrate, the resulting concentration would remain smaller than the USEPA maximum contaminant level for nitrate. Specific conductance and chloride concentrations of ground water beneath the lagoon were larger than those reported for regional ground waters of Whidbey Island (Sapik and others, 1988) but were within the range of concentrations typical for treated municipal wastewaters (National Research Council, 1996). For conservative constituents, such as chloride and specific conductance, ground water beneath the former sewage lagoon exhibited some effects of seepage from the former sewage lagoon but did not contain nitrogen concentrations greater than the maximum contaminant level.

Strongly reducing geochemical conditions in ground-water samples from beneath the former sewage lagoon were indicated by the absence of dissolved oxygen and nitrate and the presence of large concentrations of ferrous iron. Reducing conditions commonly are observed in ground waters with abundant labile organic carbon such as from peat deposits, septic systems, or sewage lagoons. Total organic carbon (TOC) concentrations measured in ground water from beneath the former sewage lagoon ranged from 13.6 to 72.5 mg/L. The largest TOC concentration was measured at the upgradient well at site LSU3. Large concentrations of TOC in ground water are characteristic of the occurrence of peat within aquifer materials. Given the abundant supply of organic carbon in ambient ground water at the site, any nitrate that may leach from residual sludge at the former sewage lagoon is expected to be quickly denitrified or transformed to nitrite or ammonia under the strongly reducing geochemical conditions that are present.

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