1. Use of a Vadose Zone Biobarrier for Removal of Nitrate from Percolating Groundwater Jim Hunter, USDA-ARS, Fort Collins, Colorado
PROBLEM Nitrate contamination of groundwater is an important agricultural problem. INTRODUCTION Biobarriers (a.k.a. permeable reactive barriers or denitrifying walls) are a promising technology for treating groundwater. In recent years these barriers have become an established treatment method for remediating aquifer water contaminated with a variety of contaminants. Horizontal or vertical barriers have been constructed by excavating an area and backfilling the area with a mixture of sand, pea gravel, and an insoluble electron donor (Figure 1). Sawdust, crop residue, cotton, newspaper and sulfur have all been used or proposed as electron donors (substrates) for biobarriers. Deeper barriers have been formed by the injection of emulsions containing vegetable oil and other nutrients into sandy aquifers. Biobarriers function by providing a microbial substrate in a permeable matrix. The substrate, usually an organic carbon, stimulates microbial activity in aquifers where microbial metabolism is normally limited by substrate availability. Increased activity results in the microbial degradation of groundwater contaminants that can act as microbial electron acceptors (i.e. nitrate, chlorate, perchlorate, and many halogenated compounds).
METHODS Columns were 10 by 145 cm PVC pipes filled with sand (Figure 2). Two sets of columns, the positive control and the treatment group, contained a 25 cm biobarrier formed by mixing 2,700 g of sand with 300 g of sawdust soaked in soybean oil (1:2, w:w). A third set of columns, the negative control, contained no biobarrier. The biobarriers in the positive control group were saturated throughout the 12-week-study while the other groups were allowed to drain. All columns were inoculated with a soil wash solution. Water containing 20 mg/L nitrate-N was periodically (Figure 3) applied to the surface of all columns and effluent samples were collected at intervals.
of vadose zone barriers is unknown. The biobarriers in our field capacity columns lost 5.6% of their organic content during the 12 week study (Table 1). At this rate the biobarrier would be depleted of organic substrate in ~3.8 years. Substrate lost from the saturated biobarriers was not statically significant.
CONCLUSIONS
The results show that vadose zone biobarriers effectively removed nitrate-N from groundwater as contaminated water percolated through the columns. In addition, the results indicate that vadose zone barriers may have a useful life of several years. Vadose zone biobarriers would be much cheaper to install than deeper saturated zone barriers and should provide a useful means of protecting aquifers from nitrate contamination. Also, vadose zone barriers could be positioned to intercept runoff protecting surface water from contamination (Figure 6).
Figure 3. Timing of water additions (arrows), amounts of water added (thickness of arrow), and moisture content of biobarriers in controls (A) and in the treatment (B).
Table 1. Biobarrier organic matter content at the start and at the end of the 12 week study.
Organic matter
Start
End
-- mg/g --
Saturated w/ biobarrier
109.5 ± 6.7
99.6 ± 0.9
• Field capacity w/o biobarrier
1.4 ± 0.0
0.3 ± 0.0
• Field capacity w/ biobarrier
103.7 ± 1.8
97.9 ± 0.9
Figure 4. Total volume of effluents collected from columns.
Figure 2. Column construction and appearance.
Figure 1. Use of biobarriers to protect an uncontaminated aquifer and to clean a contaminated aquifer.
RESULTS AND DISCUSSION
During the study 8.4 to 9.7 L of effluent were collected from each column (Figure 4). Nitrate-N in the effluents (Figure 5A & 5B) from the treatment and positive control columns declined as the study progressed and averaged 0.4 ± 0.1 and 0.8 ± 0.1 mg/L N (respectively) during the last six weeks of the study, a greater than 95% reduction in nitrate-N. In contrast, nitrates in the effluents from the negative control averaged 17.9 ± 0.4 mg/L N during the last half of the study. Regardless of treatment, only small amounts of ammonia (< 0.5 mg/L-N, and nitrite (< 0.1 mg/L-N) were present in effluents.
Moisture probes placed within the biobarriers show that the barriers removed nitrate even when the water content was in the 20 to 40% pore filled space range (Figure 3).
The life of a biobarrier usually depends upon its ability to retain sufficient substrate to stimulate microbial activity. In the saturated zone biobarriers can last for years, perhaps decades, before the substrate needs to be replenished. The life expectancy
Figure 5. Nitrate in control effluents (A) and in treatment effluent (B).
Figure 6. Vadose zone biobarriers, covered with a layer of permeable material, might be used to intercept runoff thereby protecting surface waters from nitrate contamination.
OBJECTIVE
Normally biobarriers have been employed in aquifers or nearly saturated soils. The present study used sand filled columns to evaluate the use of these barriers under shallow well-drained conditions. Evaluations examined the ability of the biobarriers to remove nitrate and anticipated barrier longevity.