Development of small outdoor microcosms for studying contaminant transformation rates and mechanisms under various water column conditions

Three outdoor microcosm treatments were developed to simulate aerobic (aero bic), thermally stratified (stratified), and anaerobic (anaerobic) water co lumns for assessing contaminant transformation rates under controlled, quas i-natural conditions. Twelve 3.2-m-diameter by l.4-m-deep fiberglass tanks (11.3 m(3) water volume) were used in the study, four for each treatment. T he tanks were variably manipulated both physically (with internal frames an d plastic layers) and chemically (with sediment trays and grass supplements ) to facilitate the three environments. The manipulations successfully crea ted three different and reproducible (both between tanks with common treatm ents and over time) aquatic systems for further study. The aerobic treatmen t produced a good simulation of a P-limited, mesotrophic water column. Alte rnately, both the stratified and anaerobic treatments developed eutrophic t o hypereutrophic water conditions and had nutritionally balanced total nitr ogen (TN) and total phosphorus (TP) levels (approximately 14:1 mg N/mg P). Mild thermal stratification was achieved in the stratified units and low-ox ygen conditions were established in the anaerobic units. A trial study, usi ng the herbicide alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanil ide], was performed to test the utility of the three microcosm designs for assessing comparative in situ contaminant transformation rates. This experi ment was initiated by adding alachlor to all tanks to a final concentration of 50 mu g/L. Alachlor levels and other chemical parameters were then moni tored for 36 d. The estimated first-order alachlor decay coefficients and h alf-lives for the aerobic, stratified, and anaerobic treatments were 0.011/ d (63 d), 0.024/d (29 d), and 0.020/d (35 d), respectively. Subsequent anal yses showed that the highest rates of alachlor decay occurred in units with grass supplements, low DO and pH levels, and comparatively high TP levels. These results suggest that lower nutrient (i.e., low TP levels), photosynt hesis-dominated aquatic systems produce lower alachlor decay rates, whereas higher nutrient, decomposition-dominated systems produce higher decay rate s. This project shows that diverse physical and chemical environments can b e created in small outdoor microcosms and that these systems can be used su ccessfully to assess contaminant decay rates in natural systems.