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HOW CAN WE ENHANCE PHOSPHORUS REMOVAL IN CONSTRUCTED FARM WETLANDS?

Constructed and restored wetlands have significant potential to reduce nutrient losses in drainage waters from New Zealand farms. While both types of wetland show reasonably good nitrogen removal efficiencies, they are not always so effective at phosphorus (P) removal and their flooded topsoils can be net sources of P. Wetland P removal efficiency could be enhanced, either by adding a P-retentive amendment to the soil in the bottom of the wetland, or installing a porous filter with a high P adsorbency and retention capacity at the end of the wetland. A review was carried out to evaluate a range of natural, processed, modified and waste materials reported in the scientific literature as having the ability to remove P from water (Ballantine and Tanner, 2010), on which this contribution is based. The reported performance of the materials reviewed varied widely. A simple scoring system based on P removal characteristics, availability, likely cost and potential reuse and ease of disposal on saturation was used to identify the materials with most promise as soil amendments for constructed wetlands. Allophane, Papakai tephra, limestone and alum were judged as materials with the most potential as soil amendments. Field and lab tests are now underway to determine which of the above materials might be most suitable to enhance P removal in constructed farm wetlands. Another possible approach is to use subsoil or a mix of subsoil and topsoil as the growth media in the base of the wetland to avoid P release on flooding of P-rich agricultural topsoils. Introduction In an attempt to reduce nutrient losses from farm drainage systems, constructed wetlands have been trialled to evaluate their ability to remove nutrients and other pollutants from drainage water (Tanner et al. 2005). While constructed wetlands are effective in removing N (entering in the form of nitrate through microbial denitrification), their capacity to remove P tends to be much lower and more variable, and they can be either sources or sinks of P (Reddy et al. 1999). In studies of natural and constructed wetlands treating diffuse agricultural run-off, retention of sediment-associated P deriving from cultivated cropland can be significant (Braskerud 2002; Mitsch et al. 1995), but removal of dissolved forms of P tends to require large wetland areas, long residence times and is more variable. In studies of constructed wetlands treating tile drainage from grazed pastures carried out in New Zealand (Sukias et al. 2006; Tanner et al. 2005), the P loads measured at the outlets of farm drainage wetlands at Toenepi and Bog Burn have remained higher than at the inflow over periods of 35 years, meaning that these wetlands have actually been net sources of P. P retention in constructed wetlands occurs through substratum adsorption, chemical precipitation, bacterial immobilisation, plant and algal uptake, incorporation into organic matter and sediment accretion (Kadlec & Knight 1996). One possible way to enhance P