Library

Notes: Nitrate is now recognized as a widespread ground water contaminant, which has led to increased efforts to control and mitigate its impacts. This study reports on the long-term performance of four pilot-scale field trials in which reactive porous barriers were used to provide passive in situ treatment of nitrate in ground water. At two of the sites (Killarney and Borden), the reactive barriers were installed as horizontal layers underneath septic system infiltration beds; at a third site (Long Point), a barrier was installed as a vertical wall intercepting a horizontally migrating septic system plume; and at the fourth site (North Campus), a barrier was installed as a containerized subsurface reactor treating farm field drainage water. The reactive media consisted of 15% to 100% by volume of waste cellulose solids (wood mulch, sawdust, leaf compost), which provided a carbon source for heterotrophic denitrification. The field trials have been in semicontinuous operation for six to seven years at hydraulic loading rates ranging from six to 2000 L/day. Trials have been successful in attenuating influent NO3- (or NO3-+ NH4+ at Borden) concentrations averaging from 4.8 mg/L N at North Campus to 57 mg/L N at Killarney, by amounts averaging 80% at Killarney, 74% at Borden, 91 % at Long Point, and 58% at North Campus. Nitrate consumption rates were temperature dependent and ranged from 0.7 to 32 mg L N/day, but did not deteriorate over the monitoring period. Furthermore, mass-balance calculations indicate that carbon consumption by heterotrophic denitrification has so far used only about 2% to 3% of the initial carbon mass in each case. Results suggest that such barriers should be capable of providing NO3- treatment for at least a decade or longer without carbon replenishment.Reactive barriers have now been used to treat nitrate contamination from a variety of sources including septic systems, agricultural runoff, landfill leachate, and industrial operations. This demonstration of successful long-term operation should allow this technology to become more widely considered for nitrate remediation, particularly at sites where passive treatment requiring a minimum of maintenance is desired.

Notes: A plume of chloride (100 to 〉 1000 mg/L) was found to extend up to 40 m laterally from the edge of a landfill located on a thick sequence of clay-rich glacial deposits. The plume is mainly confined to the seasonally saturated highly weathered and fractured zone (upper 3 to 4 m) of the deposits. In the high concentration region of the plume there was good agreement between chloride concentrations determined from piezometer water samples, which were expected to sample fracture pore water, and pore water extractions from continuous core samples, which were expected to be dominated by pore water from the fine-grained matrix. The pore water extractions provided much better lateral and vertical resolution of the plume than was obtained from the piezometer water samples. The length of the plume based on the data from the piezometer water samples was 12 m less than the length determined from the core samples, apparently because the piezometers were located above or below the relatively thin plume, or were above the water table at the time of sampling. Chloride concentrations in many of the deeper piezometers were higher than measured in core samples. This was likely due to inflow from the shallow highly contaminated zone during drilling, as indicated by declining concentration values measured in many of these piezometers over a 10-month period. The core samples were protected by a liner and were not significantly influenced by drilling-induced contamination. Downhole electrical conductivity profiles measured with a Geonics EM-39 correlated well with chloride concentration profiles determined from the core samples. Surface measurements of electrical conductivity with a Geonics EM-31 were effective at detecting the edge of the waste (which was buried under 2 m of clay), but were less effective at detecting the lateral extent of the chloride plume. Based on this and previous investigations, it is expected that an equivalent porous medium (EPM) approach will be appropriate for simulating lateral contaminant transport in the highly weathered and fractured zone. However, predictive modeling of the behavior of the chloride plume will be complicated by the presence of multiple possible sources of chloride and uncertainty concerning the hydraulic conductivity distribution, as well as by difficulty in predicting the influence of large seasonal variations in the elevation and slope of the water table.

Notes: Boreholes drilled through contaminated zones in fractured rock create the potential for vertical movement of contaminated ground water between fractures. The usual assumption is that purging eliminates cross contamination; however, the results of a field study conducted in a trichloroethylene (TCE) plume in fractured sandstone with a mean matrix porosity of 13% demonstrates that matrix-diffusion effects can be strong and persistent. A deep borehole was drilled to 110 m below ground surface (mbgs) near a shallow bedrock well containing high TCE concentrations. The borehole was cored continuously to collect closely spaced samples of rock for analysis of TCE concentrations. Geophysical logging and flowmetering were conducted in the open borehole, and a removable multilevel monitoring system was installed to provide hydraulic-head and ground water samples from discrete fracture zones. The borehole was later reamed to complete a well screened from 89 to 100 mbgs; persistent TCE concentrations at this depth ranged from 2100 to 33,000 μg/L. Rock-core analyses, combined with the other types of borehole information, show that nearly all of this deep contamination was due to the lingering effects of the downward flow of dissolved TCE from shallower depths during the few days of open-hole conditions that existed prior to installation of the multilevel system. This study demonstrates that transfer of contaminant mass to the matrix by diffusion can cause severe cross contamination effects in sedimentary rocks, but these effects generally are not identified from information normally obtained in fractured-rock investigations, resulting in potential misinterpretation of site conditions.

Notes: A small-scale field experiment was carried out to demonstrate the effectiveness of using septic system -type infiltration beds for on-site treatment of landfill leachate. Using an infiltration bed with a 3-m-thick vadose zone of medium sand, and loaded at a rate 01 18 cm/day, a treatment efficiency of 〉99 percent was obtained for Fe, 94 percent for NH4+, and 54 percent for dissolved organic carbon (DOC). Attenuation occurred during one- to two-day residency in the aerobic vadose zone (pore gas O2 〉 12 percent by volume) as a result of oxidation reactions that caused nitrification of NH4+, convened Fe2+ to Fe3+ allowing subsequent precipitation of sparingly soluble Fe oxyhydroxide minerals, and biodegraded DOC. Attenuation of an aerobically degradable trace volatile organic compound (dichlorobenzene) was also noted, although other less degradable compounds (trichloroethylene and tetrachloroethylene) persisted.Fe mineral precipitation caused a discontinuous hardpan layer to occur in the zone immediately below the infiltration pipes. However, this layer did not become impermeable or continuous enough to significantly impede infiltration during the 82-day experiment.Advantages of this technology for leachate treatment are that it is low cost, it is simple to construct and operate. and treatment occurs on-site, avoiding the cost of transporting leachate off-site for treatment.