The seasonal pattern of uptake of pollutants is very important when considering the use of wetlands to treat urban stormwater. Most authors agree that, during the growing season, the uptake of nutrients and pollutants by the vegetation from both water and sediments is high (Dinges, 1982; Nichols, 1983; Mitsch and Gosselink, 1986; Van der Valk et.al., 1987; Christian, 1990; Radoux and Kemp, 1990). What is not so clear is the position in the autumn and winter when the vegetation is senescent.
Research by Mitsch and Gosselink (1986) found that a wetland may be both a sink for an inorganic form of a nutrient and a source for an organic form of the same nutrient. They found
that levels of nitrogen and phosphorus were low in the summer, when uptake by plants was at its peak, and high in the winter when the plants were dormant. They also found that the storage of nutrients in vegetation was seasonally partitioned into above ground and below ground stocks.
Van der Valk et al. (1987) found that nitrogen and phosphorus were returned to the water by macrophytes and epiphytes when the plants started to decay in the autumn. The plants released the nitrogen by leaching, litter fall and root excretion. The amount of nitrogen released in this way depended upon the anatomy and morphology of the plant species and the related pattern of litter fall. Some of the phosphorus entered the sediments and some the litter layer. Although phosphorus leached from fresh litter, the older
litter accumulated it, due to the activity of microbes.
Shutes et al (1993) reported seasonal variations in Tvoha tissue copper and zinc concentrations, with peak levels coinciding with the growth period in June and July, when there was an increase in demand for these plant micro nutrients. They also found a bioaccumulation of copper in root tissues between February and April followed by a fall in concentrations from April to June, when the rate of root growth exceeded the rate of uptake of copper. The maximum lead and cadmium tissue levels were found to occur during August and October. This, they suggested, was possibly due to reduced competition from copper and zinc and to translocation from root tissue being augmented by atmospheric deposition as well as by increases in the permeability of the leaves during decomposition.
Christian (1990) found that there was a seasonal dimension to the treatment of gravel beds planted with reeds, with a better treatment function provided in the spring and summer periods. He thought that this might be due to the reeds' growth pattern with, possibly, the temperature and the water level also having a role. The bed planted with the fewest reeds showed the smallest seasonal variation, suggesting that the role of the reeds was an important one.
Nichols (1983) quotes studies undertaken in the United States that showed, for example, a retention of 83% of phosphate during the summer from cattail marshes but only 1% in the autumn. He estimated that when the wetland vegetation died back between 35 and 75% of plant tissue phosphorus, and slightly less nitrogen, was released back into the water. Radoux and Kemp (1990) examined the impact of seasonal changes on water purification by Typha latifolia. They compared planted and unplanted systems and found that the retention rates fluctuated seasonally (Table 3.3):
Table 3.3 Seasonal Percentage Pollutant Retention Rates
Parameter Cascade Spring Summer Autumn Winter Total
S.S Typha Sand 93.0 15.4 94.3 25.0 81.9 36.3 88.7 79.4 88.1 51.2 Total COD Typha
Sand 82.6 13.9 87.3 23.3 69.5 47.7 69.9 65.8 73.9 41.0 Total N Typha Sand 83.7 38.3 87.8 47.7 53.1 34.4 24.2 18.9 65.2 42.3 Total P Typha Sand 75.6 28.2 83.7 44.0 21.3 13.0 0.6 - 0.3 50.4 27.4 Source: Radoux and Kemp (1990)
Table 3.3 shows that the performance efficiency of the planted cascade was much better than that of the unplanted cascade. It also clearly indicates that, although the suspended sediment and total COD levels were not subject to dramatic seasonal variation, the total nitrogen and total phosphorus levels did show a seasonal pattern, with lower treatment efficiencies achieved in the autumn and winter. The reason for this would seem to be the influence of the vegetation and the ambient temperature.
Temperature has a significant influence on microbiological activity with a 20% reduction in activity reported in the winter (Brix, 1987b). The degradation of organic matter by micro-organisms and the processes of nitrification and
denitrification are all influenced by temperature. According to Brix (1987b) the optimum temperature for nitrification to take place is between 25 and 35°C. When the temperature falls below 5°C nitrification progresses extremely slowly. Reed et al (1995) state that at below 10°C nitrification is strongly temperature dependent and at 0°C nitrification ceases altogether. This is illustrated by an example they give from a wetland in Iowa, USA, that received an influent ammonia concentration of 16 mg/1 and had a hydraulic residence time of approximately 14 days (Diagram 3.12)