Abstract:
This study was conducted to investigate the capacity of Cynodon nlemfuensis (star grass) to accumulate lead (Pb) and cadmium (Cd) and develop metal uptake models for sandy soils receiving treated sewage from domestic and industrial sources. The study area comprised a non-polluted area and an adjacent area that received treated sewage from Harare’s Firle Wastewater Treatment Plant for over 30 years. Measured soil properties, total Pb and Cd in soils and grass and past records of Pb and Cd in treated sewage were analysed. Growing grass in a greenhouse in pots with previously non-polluted soils amended by single and mixed Pb and Cd salts and irrigated with treated sewage tested the uptake capacity of star grass. Yields, soil bio-available and grass Pb and Cd levels were measured and used to develop models for estimating critical soil and grass concentrations at which productivity declines. In the field, star grass grown in 10m x 10m plots in the non-irrigated and irrigated areas, received varying amounts of treated sewage over 11 months. Soil bio-available and grass metal contents were measured and used to develop field-based models to predict Pb and Cd content in star grass. Star grass had a high Pb and Cd extraction capacity, making it unsuitable for pasture if grown on polluted soils. Correlation between total Pb and Cd in soils and grass was insignificant (p<0.05). Logarithm-based models of log10 bio-available soil levels and log10 grass metal levels provided the best-fit regression models for Pb and Cd predictions in grass. Toxicity levels of Pb and Cd that were derived for star grass from pot-based models were higher than levels recommended for pasture. Toxicity occurred without visible signs on grass, making it difficult to recognise toxicity without testing. The field-based uptake models predicted safe bio-available limits for pasture on sandy soils. The co-presence of Pb and Cd resulted in increased Cd uptake but did not significantly affect Pb uptake. Star grass can accumulate more than 1 mg/kg of Cd at total soil Cd levels of less than 1 mg/kg, suggesting that the soil limit may be too high for a sandy soil.
