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

Abstract

AbstractThree outdoor microcosm treatments were developed to simulate aerobic (aerobic), thermally stratified (stratified), and anaerobic (anaerobic) water columns for assessing contaminant transformation rates under controlled, quasi‐natural conditions. Twelve 3.2‐m‐diameter by 1.4‐m‐deep fiberglass tanks (11.3 m3 water volume) were used in the study, four for each treatment. The tanks were variably manipulated both physically (with internal frames and plastic layers) and chemically (with sediment trays and grass supplements) to facilitate the three environments. The manipulations successfully created three different and reproducible (both between tanks with common treatments and over time) aquatic systems for further study. The aerobic treatment produced a good simulation of a P‐limited, mesotrophic water column. Alternately, both the stratified and anaerobic treatments developed eutrophic to hypereutrophic water conditions and had nutritionally balanced total nitrogen (TN) and total phosphorus (TP) levels (approximately 14:1 mg N/mg P). Mild thermal stratification was achieved in the stratified units and low‐oxygen conditions were established in the anaerobic units. A trial study, using the herbicide alachlor [2‐chloro‐2′,6′‐diethyl‐N‐(methoxymethyl)acetanilide], was performed to test the utility of the three microcosm designs for assessing comparative in situ contaminant transformation rates. This experiment was initiated by adding alachlor to all tanks to a final concentration of 50 μg/L. Alachlor levels and other chemical parameters were then monitored for 36 d. The estimated first‐order alachlor decay coefficients and half‐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 analyses 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), photosynthesis‐dominated aquatic systems produce lower alachlor decay rates, whereas higher nutrient, decomposition‐dominated systems produce higher decay rates. This project shows that diverse physical and chemical environments can be created in small outdoor microcosms and that these systems can be used successfully to assess contaminant decay rates in natural systems.