Nitrate Removal Rates in a 15‐Year‐Old Permeable Reactive Barrier Treating Septic System Nitrate

Abstract

Permeable reactive barriers (PRBs) have gained popularity in recent years as a low-cost method for ground water remediation. However, their cost advantage usually requires that these barriers remain maintenance free for a number of years after installation. In this study, sediment cores were retrieved from a pilot-scale PRB consisting of a sand and wood particle (sawdust) mixture that has been in continuous operation for 15 years treating nitrate from a septic system plume in southern Ontario (Long Point site). Reaction rates for the 15-year-old media were measured in dynamic flow column tests and were compared to rates measured in year 1 using the same reactive mixture. Nitrate removal rates in the 15-year-old media varied, as expected, with temperature in the range of 0.22 to 1.1 mg N/L/d at 6°C to 10°C to 3.5 to 6.0 mg N/L/d at 20°C to 22°C. The latter rates remained within about 50% of the year 1 rates (10.2 6 2.7 mg N/L/d at 22°C). Near the end of the year 15 column test, media particles >0.5 mm in diameter, containing most of the wood particles, were removed from the reactive media by sieving. Nitrate removal subsequently declined by about 80%, indicating that the wood particles were the principal energy source for denitrification. This example shows that some denitrifying PRBs can remain maintenance free and be adequately reactive for decades.

Long-term nitrate removal in a denitrification wall

“…Since Robertson et al (2008) presents the only decadal study on denitrification walls, long-term studies of denitrification wall performance remain lacking. More information on sustainability is needed including bioreactors utilizing different C media and bioreactors installed in different climates, soils, and groundwater systems.…”

“…To date, Robertson et al (2008) presents the only decadal study of the performance of denitrification walls. This study revisited the wall installed downgradient of a septic tank in Canada after 15 years of use (Robertson and Cherry, 1995).…”

“…Denitrification walls are low-cost and low-maintenance, but a key factor in terms of wide spread adoption is how long they will be effective at NO 3 -removal. They have been shown to continue to remove NO 3 -for 7 years (Robertson et al, 2000;Schipper and Vojvodic-Vukovic, 2005), 9 years (Moorman et al, 2010), and in the only decadal study, up to 15 years (Robertson et al, 2008). Robertson et al (2008) took cores from the denitrification wall and then ran NO 3 -through them in a lab; a full field sampling was not undertaken.…”

Implications of non-equilibrium transport in heterogeneous reactive barrier systems: Evidence from laboratory denitrification experiments

“…These reactive materials were selected as substrates for the growth of denitrifying bacteria in a field-scale barrier system (Herbert and Björnström, 2009); other denitrification studies have shown that sawdust is a suitable substrate (e.g. Robertson et al, 2008;Schipper et al, 2004), providing a long-term carbon source for the growth of heterotrophic denitrifying bacteria. The sewage sludge is included because it is a source of denitrifying bacteria and allows for the rapid establishment of a denitrifying community in the columns.…”

“…chlorinated solvents, nitrate), reactive materials generally consist of electron donor sources such as zerovalent iron (Fe 0 ) or organic carbon (Blowes et al, 2000). Organic substrates have been chosen in a variety of cases (Benner et al, 2002;Blowes et al, 1994;Robertson et al, 2008;Schipper et al, 2004) where carbon sources are required for heterotrophic microbial reduction of groundwater constituents (e.g. dissimilatory sulfate reduction, denitrification).…”