Biodegradability and Toxicity of Hydrocarbon Leachate from Land Treatment Units

The biodegradability of leachate from the land treatment of hydrocarboncontaminated soil was investigated in the laboratory using respirometry and toxicity test­ ing in combination with total petroleum hydrocarbon (TPH) measurements. Soil in land treatment units (LTU) had been contaminated with a diesellike hydrocarbon mixture formerly used as a diluent for crude oil at an oil field in California. Leachate was col­ lected from two different LTUs for treatability testing in a respirometer under aerobic conditions. Only about 12% reduction in TPH concentration was observed after aeration for 161 days, indicating limited biodegradability of the hydrocarbon constituents in the leachate. Similarly, Microtox toxicity did not change after 130 days. Leachate bio­ degradability was further tested by comparison to diluent-contaminated groundwater from the same site. Leachate diluted to the same TPH concentration as the contaminated groundwater was three times less toxic, but was much less biodegradable. The recalci­ trance of the leachate hydrocarbons may be attributable to their high molecular weight, since the majority of the TPH was long-chained hydrocarbons of C20 or greater for leach­ ate. In contrast, the diluent contaminated groundwater has a majority of its TPH concentration in short-chained hydrocarbons of C20 or less, which were more easily bio­ degraded. These short chain hydrocarbons are typically more toxic than the longer chain hydrocarbons, which would explain the observed decrease in toxicity of the diluent­ contaminated groundwater during biodegradation. INTRODUCTION Soil at the former Guadalupe Oil Field was contaminated with a diesel-range hydrocarbon mixture that was used as a diluent for facilitating pumping the viscous crude oil at the site. Soil from heavily contaminated sites near the ocean has been excavated and options are currently being explored for treatment and/or disposal of this soil. One important option is on-site biological treatment using land farming. Pilot-scale land treat­ ment units (LTU) have been operated on-site to test this option of soil treatment. These pilot studies indicate that the leachate from the LTU contains hydrocarbon contaminants that might have a detrimental effect on the groundwater. Thus it is important to under­ stand the fate, transport and toxicity of this leachate. The purpose of this research is to determine the biodegradability of leachate from pilot-scale LTUs and to determine if the leachate toxicity is reduced by biodegradation. The composition of the diluent contamination at the Guadalupe site has been described by Haddad and Stout (1996). Approximately 90% of the diluent at Guadalupe is comprised of hydrocarbons with an equivalent carbon range of 14 to 30. Forty-one different polycyclic aromatic hydrocarbons (PAHs) have been identified, and the domi­ nant family of PAHs in the diluent are naphthalenes. The leachate from a LTU was previously shown to have measurable toxicity (CH2M Hill, 2001 and Coffey 2002). The Microtox test indicated EC50 values of 3.3 to 6.1%. LTU leachate was found to be highly polar (approximately 74%–100% polar) after about 100 days of tilling, watering and nutrient addition (Coffey, 2002). These polar compounds could contribute to leachate toxicity. PAHs are also toxic (Kropp and Fedorak, 1998), and may contribute to the toxicity of the leachate. Leachate biodegradability and toxicity were investigated in two laboratory treata­ bility experiments. In the first experiment biodegradability and changes in toxicity were determined for leachate from a land treatment unit (LTU3) with soil exhibiting high total petroleum hydrocarbon (TPH) concentrations. Biodegradability was determined by measuring respiration rates in a respirometer and by measuring TPH concentrations initially and at 51 days and 161 days. The second experiment was used to compare the biodegradability and toxicity of leachate with that of diluent-contaminated groundwater of similar TPH concentration. For this second experiment leachate from Land Treatment Unit 2 (LTU2) was used, which had significantly lower soil TPH concentrations than LTU3. Toxicity in both experiments was estimated using the Microtox test. MATERIALS AND METHODS The leachate for Experiment 1 was collected from lysimeters on LTU3. The test plot at LTU3 had been tilled on a bi-weekly basis and nutrients had been added. The leachate for Experiment 2 was generated by leaching fresh water through diluent­ contaminated soil from Land Treatment Unit 2 (LTU2). Samples from each LTU site were incubated at 25°C under controlled laboratory conditions while measuring either O2 consumption or CO2 production with a respirometer. Microtox toxicity was determined for initial samples and periodically during biodegradation in duplicate or triplicate to determine if the toxicity was decreasing. The TPH concentration of the leachate was ana­ lyzed for initial and final samples using gas chromatography/mass spectroscopy (GC/MS) by Zymax Envirotechnology to determine if the hydrocarbons were biodegrading. For Experiment 1 with LTU3 leachate, respiration was measured for triplicate 2-L samples of leachate and one control of San Luis Obispo tap water. Two liters of sample were used to provide two 1-L samples for TPH analyses at 51 days and 161 days. No nutrients were added for this test to observe biodegradation in an unamended state. In Experiment 2 biodegradation and toxicity of leachate was compared to that of diluent-contaminated groundwater. Leachate with 24 mg/L TPH was diluted to have the same initial TPH concentration as the comparison diluent-contaminated groundwater samples. Inoculum was not added to either the leachate or groundwater samples during this experiment. Nutrients were added to both the leachate and contaminated groundwater to ensure adequate nutrient availability. The Microtox test (Strategic Diagnostics Inc., Newark DE) was used for all toxicity assays in these experiments. The Microtox test software calculates the results of the test as EC50. The Microtox EC50 is the effective concentration for which 50% of the bioluminescence of the test bacterium (Vibrio fischeri) is extinguished by toxicity. RESULTS AND DISCUSSION Biodegradability and Toxicity of Leachate from LTU3. The initial TPH concentration of the triplicate leachate samples was 96 ±4 mg/L (Table 1). Hydrocarbon biodegradation TABLE 1. LTU3 leachate TPH degradation. Percent TPH TPH Concentration (mg/L) Degraded Day 0 Day 51 Day 161 (161 days) Sample 1 98 74.9 72 26.53 % Sample 2 92 98.9 86 6.52 % Sample 3 100 89 96 4.00 % was very slow for the 161 days of this experiment. The average TPH degradation was 12% and the standard deviation was ±12%. The majority of TPH biodegradation appears to have been for the C-18 to C-24 range (data not shown). No decrease in Microtox toxicity was observed with for the 130 days (Figure 2). Initially EC50 for the leachate was 9%. Oxygen uptake was significant after an initial 10-day lag phase (Figure 1). The very high TPH values in LTU3 leachates are indicative of separate phase product in the sample. A cc u m u la ti ve O xy g en C o n su m p ti o n (m g /L ) 250 Microtox ® 14