Microtopographic modification conserves urban wetland water quality by increasing the dissolved oxygen in the wet season

Microtopography affects hydrological processes and forms different microhabitats.Our previous study uncovered that riparian zone microtopography created various microhabitats with different soil environments and runoff-infiltration patterns.However,how riparian microtopography and microtopography within the water area(waterfall and tributary)affects downstream water quality remains unclear.Therefore,water samples were taken almost monthly in both the main stream and the tributary,before and after waterfalls,and near the bottom of three microtopographic types from June 2016 to March 2017.Compared with the dry season,the fact that water quality worsened in the wet season and that there were positive correlations for nitrate(NO3-)between water and the corresponding soil samples suggested that the riparian-soil environment affected the adjacent water quality mainly in the wet season.Nevertheless,riparian microtopography did not influence water quality downstream because of the low rainfall frequency and the weak leaching process due to plant interception.In the wet season,both the tributary and the waterfall increased the dissolved oxygen in the water body and,therefore,lowered the risk of eutrophication.The tributary has two pathways for improving the water quality,by increased disturbance and flow velocity,while the waterfall only has the former.However,such effects were not significant in the dry season.We conclude that the application of microtopographic modification is useful in maintaining urban wetland water quality in wet seasons.

Performance of pond–wetland complexes as a preliminary processor of drinking water sources

Shijiuyang Constructed Wetland（110 hm^2） is a drinking water source treatment wetland with primary structural units of ponds and plant-bed/ditch systems. The wetland can process about 250,000 tonnes of source water in the Xincheng River every day and supplies raw water for Shijiuyang Drinking Water Plant. Daily data for 28 months indicated that the major water quality indexes of source water had been improved by one grade. The percentage increase for dissolved oxygen and the removal rates of ammonia nitrogen, iron and manganese were 73.63%, 38.86%, 35.64%, and 22.14% respectively. The treatment performance weight of ponds and plant-bed/ditch systems was roughly equal but they treated different pollutants preferentially. Most water quality indexes had better treatment efficacy with increasing temperature and inlet concentrations. These results revealed that the pond–wetland complexes exhibited strong buffering capacity for source water quality improvement. The treatment cost of Shijiuyang Drinking Water Plant was reduced by about 30.3%. Regional rainfall significantly determined the external river water levels and adversely deteriorated the inlet water quality, thus suggesting that the ＂hidden＂ diffuse pollution in the multitudinous stream branches as well as their catchments should be the controlling emphases for river source water protection in the future. The combination of pond and plant-bed/ditch systems provides a successful paradigm for drinking water source pretreatment. Three other drinking water source treatment wetlands with ponds and plant-bed/ditch systems are in operation or construction in the stream networks of the Yangtze River Delta and more people will be benefited.