Phytoremediation of nitrate contaminated soil and groundwater by desert phreatophytes in Monument Valley, Arizona

摘要

The results of a study on phytoremediation of nitrate and ammonium contaminated soil and groundwater at the Monument Valley Uranium Mill Tailings Remedial Action (UMTRA) Project site are presented in this dissertation. The following is a summary of the findings: A phytoremediation plot was established to remediate soil nitrate and ammonium contamination at a former uranium ore-processing site. Atriplex canescens were planted and deficit irrigated, preventing recharge into the shallow aquifer. Initial soil concentration of nitrate-N was approximately 180 mg kg⁻¹ but decreased to only 80 mg kg⁻¹ after 41 months. A decrease in nitrate-N concentrations was observed throughout the 4.6 m soil profile, which was a unique observation. Ammonium-N concentrations remained at initial levels of approximately 180 mg kg⁻¹ and did not decline over the study period. Soluble salts decreased only 20% in the soil, which was attributable to nitrate loss. Residual soil nitrogen became enriched in ¹⁵N, indicating biological denitrification. Nitrate-N loss was 1,360 kg ha⁻¹yr⁻¹, which is approximately three times higher than agricultural soils. These findings may provide a low-cost method for soil nitrate remediation. A. canescens and Sarcobatus vermiculatus are native phreatophytic shrubs at the UMTRA Project site that were evaluated for their potential to remove nitrate from the shallow aquifer. Stable isotope signatures from plant stem water were similar to the groundwater isotope signatures, suggesting the plants are rooted in the plume. Currently only 7% of the plume area is vegetated, mainly due to heavy grazing. When protected from grazing, plants increased in cover by over 50% per year during a three-year period. Transplants of A. canescens that were protected from grazing and irrigated during the first summer after planting were rooted in the plume within three years, growing greater than 2-m in height. Based on these results, the nitrate plume could be removed within 13 years if grazing were restricted and vegetation were to be enhanced to 50% cover, whereas almost six decades would be required for remediation under current conditions. This study shows that phytoremediation may be an inexpensive and non-invasive means of nitrate remediation at this and other arid locations.