In situ Mixing of Organic Matter Decreases Hydraulic Conductivity of Denitrification Walls in Sand Aquifers

Abstract

In a previous study, a denitrification wall was constructed in a sand aquifer using sawdust as the carbon substrate. Ground water bypassed around this sawdust wall due to reduced hydraulic conductivity. We investigated potential reasons for this by testing two new walls and conducting laboratory studies. The first wall was constructed by mixing aquifer material in situ without substrate addition to investigate the effects of the construction technique (mixed wall). A second, biochip wall, was constructed using coarse wood chips to determine the effect of size of the particles in the amendment on hydraulic conductivity. The aquifer hydraulic conductivity was 35.4 m/d, while in the mixed wall it was 2.8 m/d and in the biochip wall 3.4 m/d. This indicated that the mixing of the aquifer sands below the water table allowed the particles to re‐sort themselves into a matrix with a significantly lower hydraulic conductivity than the process that originally formed the aquifer. The addition of a coarser substrate in the biochip wall significantly increased total porosity and decreased bulk density, but hydraulic conductivity remained low compared to the aquifer. Laboratory cores of aquifer sand mixed under dry and wet conditions mimicked the reduction in hydraulic conductivity observed in the field within the mixed wall. The addition of sawdust to the laboratory cores resulted in a significantly higher hydraulic conductivity when mixed dry compared to cores mixed wet. This reduction in the hydraulic conductivity of the sand/sawdust cores mixed under saturated conditions repeated what occurred in the field in the original sawdust wall. This indicated that laboratory investigations can be a useful tool to highlight potential reductions in field hydraulic conductivities that may occur when differing materials are mixed under field conditions.

Long-term nitrate removal in a denitrification wall

“…Depth to the water table should also be noted during the site analysis as it is unlikely to be cost effective to place the C material deeper than a few meters (Schipper et al, 2010a). This may have been a result of mixing of saturated sands during construction of the denitrification wall which caused a decline in aquifer hydraulic conductivity (Barkle et al, 2008).…”

Constructal design of permeable reactive barriers: groundwater-hydraulics criteria

“…2, the interception flow rate between F 1 H 1 E 1 and F 2 H 2 E 2 is Q. 4 Obviously, far downstream of the PRB (points E 1 , E 2 ), the width of the capture zone is d = Q/V 0 .…”

“…Continuous seepage of groundwater containing suspended fine particles spontaneously clogs zones of artificially increased conductivity characterized by increased Darcian velocity even without any chemical reactions (e.g., well gravel packs [3]). Chemical reactions and/or bacterial growth in PRBs cause fouling [4][5][6]. Consequently, the whole PRB interior or its periphery change their hydraulic properties during operation.…”

In Situ Batch Denitrification of Nitrate-Rich Groundwater Using Sawdust as a Carbon Source—Marydale, South Africa

“…Successful denitrification of groundwater has been practised in New Zealand for more than a decade; however, their newest attempted denitrification of non-point sources of nitrate from shallow groundwater failed due to hydraulic constraints on the performance of their denitrification wall (Schipper et al 2004). Hence, the hydraulic properties of both the wall and aquifer are integral properties to be monitored and assessed throughout the life of an operation (Barkle et al 2008). The aquifer hydraulic conductivity was reduced by a great percentage during construction so that groundwater flow occurred under the wall rather than through it (Barkle et al 2008;Schipper et al 2004).…”

“…Hence, the hydraulic properties of both the wall and aquifer are integral properties to be monitored and assessed throughout the life of an operation (Barkle et al 2008). The aquifer hydraulic conductivity was reduced by a great percentage during construction so that groundwater flow occurred under the wall rather than through it (Barkle et al 2008;Schipper et al 2004). Particle size within the aquifer is an important parameter to evaluate and, more importantly so, the redistribution thereof during construction.…”