Anne M. Hansen

Mineralization of atrazine in agricultural soil: Inhibition by nitrogen

Microbial mineralization of atrazine was characterized in soils and liquid media in the presence of nitrogen fertilizer concentrations representing typical field applications. The mineralization of atrazine in soils varied between 6 and 99% after 18 d of incubation. Half-lives of between 0.99 and more than 18 d were obtained. Mineralization kinetics and degree are related by a reciprocal trend to concentrations of available nitrogen in the soil. In liquid media, half-lives were calculated as 0.12 d in the absence of fertilizer nitrogen and as 79 d in the presence of 1,000 mg/L of KNO3-N. Only 20% of atrazine was mineralized after 18 d of incubation in the presence of this concentration of KNO3-N, whereas greater than 90% mineralization occurred after 2 d of incubation in liquid medium without KNO3-N. The results demonstrate that the mineralization of atrazine is inhibited even at fertilizer nitrogen levels lower than typical field applications. Inhibition in soil is lower than that in liquid medium, possibly because of the higher complexity of the soil system. This may explain why atrazine that infiltrates to the groundwater is persistent. The microbial consortium of the soils was characterized, and seven species were identified. The degrading capacity of these species suggests that only three species are involved in the degradation of atrazine.

Laboratory test system to measure microbial respiration rate

This study reports a sensible, accurate and economic method for continuous measuring microbial respiration. The measuring principle is an open system, with a continuous air-flow through. Evolved CO2 is absorbed and precipitated as carbonate by a Ba(OH)2 solution, causing a stoichiometrical decrease in ionic strength of the solution and in electrical conductivity. Conductivity and Ba(OH)2 concentration correlate over a range of more than three orders of magnitude with a determination coefficient of r2 = 0.999. Between 20°C and 50°C and Ba(OH)2 concentrations of up to 0.099M, an automated temperature correction was developed. The system detects evolved CO2 quantitatively up to a maximum of 0.22 mmol O2 min-1 (825 mbar, 20°C) before limiting microbial respiration. A maximum CO2 flow of 1.06 mmol min-1 is quantitatively absorbed under these conditions. The method was applied to characterise soil respiration of a soil sample from an agricultural experimental site in Tabasco, Mexico.

Modeling cadmium concentration in water of Lake Chapala, Mexico

Abstract.Watershed management in Mexico is generally carried out based on hydrological information, whereas water quality data are mainly used in the control of wastewater discharges and water uses. Information on toxic contaminants is scarce and expensive to gather. A method was developed to predict inorganic contaminant concentration in Lake Chapala, Mexico, by using hydrological data for water balance, water quality information and analyses, sediment quality monitoring data, and applying a chemical equilibrium model to estimate the possibility of increase in cadmium concentration. It is shown that decreasing water levels in Lake Chapala can lead to a significant increase in dissolved cadmium that, in the future, may affect the lake ecosystem and the water quality for different uses. Watershed managers can use this method with one by van Afferden and Hansen (2004) as an early warning system to reduce the risk that contaminant concentrations increase in the lake.

Forecast of lake volume and salt concentration in Lake Chapala, Mexico

Abstract.Using historical hydrological information and water quality records, an early warning system was developed to forecast short- and long-term trends in water level changes and volume-dependent major ion concentrations for Lake Chapala, Mexico. The developed tool allows one to forecast yearly minimum lake-water volumes with a 90% probability and less than ±5% error. The long-term behavior of lake volume and its trend towards a new equilibrium was estimated by a second empirical approach, where water storage variations were calculated by estimating changes in total lake evaporation losses due to variations in lake area. Available historical water quality records were analyzed and, by applying subsequent statistical data filtration, volume and time dependent trends in major ion concentrations were established. Mass and charge balances were applied to complete the data-sets with concentrations of ions that are not measured during ordinary monitoring. Concerning water quality regulations, minimum lake volumes were proposed to guarantee the intactness of this important drinking water resource. Despite the uncertainties of these approaches, the early warning system presented provides a useful tool for short and long-term prediction of lake water storage and quality that may support management decisions related to the whole watershed.

Internal phosphorus load in a Mexican reservoir: Forecast and validation

To determine the internal phosphorus load (IPL) as a function of redox potential (Eh) in a Mexican reservoir, the results from a phosphorus (P) release experiment were extrapolated to temporal and spatial variations of Eh in sediments, and an IPL-Eh of 24.2 ± 2.5 t/yr was obtained. This result is compared with the P mass balance (MB) in the reservoir, where the IPL-MB is determined as the difference between P inputs to the reservoir and the outputs. Inputs of P are the sum of the external P load from the hydrological basin, the IPL, and P in atmospheric precipitation; outputs of P are the sum of sedimented P, and the removal of P in water and biomass, and the resulting IPL-MB, is 26.4 ± 4.9 t/yr. In addition, P concentrations in sediment cores (SCs) are analyzed, and the historical release of P from sediments determined, resulting in an IPL-SC of 23.5 ± 1.4 t/yr. The different IPL results are similar, as average values are within the standard deviation of IPL-MB. It is concluded that analysis of the variations in Eh in sediments allows determination of the reservoirs IPL. Six-weekly IPL-Eh and IPL-MB values are analyzed, and it can be seen that IPL occurs mainly during the period from May to August, when the water column is thermally stratified.