Nitrate Reduction in an Unconfined Sandy Aquifer: Water Chemistry, Reduction Processes, and Geochemical Modeling

Abstract

Nitrate distribution and reduction processes were investigated in an unconfined sandy aquifer of Quaternary age. Groundwater chemistry was studied in a series of eight multilevel samplers along a flow line, deriving water from both arable and forested land. Results show that plumes of nitratecontaminated groundwater emanate from the agricultural areas and spread through the aquifer. The aquifer can be subdivided into an upper 10-to 15-m thick oxic zone that contains 02 and NO•-, and a lower anoxic zone characterized by Fe2+-rich waters. The redox boundary is very sharp, which suggests that reduction processes of 02 and NOff occur at rates that are fast compared to the rate of downward water transport. Nitrate-contaminated groundwater contains total contents of dissolved ions that are two to four times higher than in groundwater derived from the forested area. The persistence of the high content of total dissolved ions in the NO•--free anoxic zone indicates the downward migration of contaminants and that active nitrate reduction is taking place. Nitrate is apparently reduced to N2 because both nitrite and ammonia are absent or found at very low concentrations. Possible electron donors in the reduced zone of the aquifer are organic matter, present as reworked brown coal fragments from the underlying Miocene, and small amounts of pyrite at an average concentration of 3.6 mmol/kg. Electron balances across the redoxcline, based on concentrations of 02, NO•-, SO¬-and total inorganic carbon (TIC), indicate that pyrite is by far the dominant electron donor even though organic matter is much more abundant. Groundwater transport and chemical reactions were modeled using the code PHREEQM, which combines a chemical equilibrium model with a one-dimensional mixing cell transport model. Only the vertical component of the water transport was modeled since, in contrast to rates along flow lines, the vertical rates are close to constant as required by the one-dimensional model. Average vertical transport rates of water in the saturated zone were obtained by tritium dating. The modeling process is a two-step procedure. First the sediment column is initialized with natural water containing only oxygen as electron acceptor, and subsequently agricultural waters containing both oxygen and nitrate are fed into the column. The nitrate concentration of agricultural waters entering the saturated zone varies with time, and an input function was therefore constructed by linear mixing of natural waters and agricultural waters. This input function was fed into the column initialized with natural water, and the model run forward in time to the year 1988 where field data are available. Comparison with field data shows that the variation in groundwater chemistry is well described by the model when reduction of oxygen and reduction of nitrate by pyrite oxidation are the only redox reactions occurring. Finally, predictions are made for the distribution of water chemistry in the year 2003. Downward progression of the redoxcline is accelerated by a f...

Identification of abiotic and biotic reductive dechlorination in a chlorinated ethene plume after thermal source remediation by means of isotopic and molecular biology tools

“…3A , which is supported by the presence of pyrite in sandy aquifers in Jutland (Postma et al, 1991). Unlike in 2006, in 2014, mixed redox conditions are generally observed at the local scale.…”

“…2). Low amounts of organic matter as well as high levels of iron are expected in aquifers in this part of Jutland (Postma et al, 1991). An average groundwater velocity of 0.24 m·day -1 was previously estimated , though this may vary locally due to different hydraulic conductivities resulting from the large grain size distribution.…”

Regional analysis of groundwater nitrate concentrations and trends in Denmark in regard to agricultural influence

“…In the groundwater aquifers, organic matter, ferrous ions and pyrite (Postma et al, 1991) are the dominating nitrate-reducing agents causing the denitrification processes that take place in the nitrate-reducing anoxic zone.…”

Threshold values and management options for nutrients in a catchment of a temperate estuary with poor ecological status

“…The resulting model calculated threshold value of TN in the root zone and aerobic groundwater at and below a depth of one meter is 6.0 mg l −1 (equivalent to 26.5 mg l −1 NO − 3 ) as an average for the entire catchment area (Table 7). However, the threshold value for TN under agricultural fields can be allowed to be higher (7.4 mg l −1 , equivalent to 32.7 mg l −1 NO − 3 ) because approximately one third of the catchment area is in a non-agricultural land cover category, with a low background concentrations of TN in groundwater (< 1 mg l −1 in some areas, Postma et al, 1991) and streams (approximately 1.2 mg l −1 ) . As phosphorus is derived via many hydrological pathways (leaching, erosion, and surface runoff) to surface waters , it is not possible to calculate a groundwater P threshold value with our current knowledge.…”