By Rich McLaughlin, Ph.D
It is no secret that our population is increasingly located in urban areas in and around cities, which in turn were often established adjacent to streams and rivers as a source of fresh water. This has not been a good thing for the fresh water source in most cases. Wastewater, polluted runoff, and industrial discharges, coupled with water withdrawals, combined to greatly degrade the surface waters passing through the cities. This can be particularly acute in semi-arid areas with seasonal low flows.
The sources of pollution in an urban river were studied for three years in Denver, Colorado, USA.1 The South Platte River passes through the city and flows are lowest during the fall and winter months, when up to 90% of the flow is made up of discharges from several wastewater treatment plants. Sampling points were established upstream of a reservoir where the river emerges from the mountains, considered the non-urban control point, and at 13 additional locations in the city up to 63 km downstream. In addition, all outfalls to the river were identified, including streams, storm drains and other discharges. Samples were analyzed for nutrients, biological oxygen demand, dissolved oxygen, coliform bacteria, pH, and temperature.
There are two wastewater treatment plants discharging to the South Platte via tributaries, and it was assumed they would be the major source of pollutants during the dry periods. While they were contributors, there were other “hot spots” detected. One was an area of concentrated storm drains, roughly 11 per km, and the other was an area with concentrated industries. Peaks of 25.8 mg L-1 nitrate-N and 14.3 mg L-1 orthophosphate-P were found in the winter. Overall, a significant amount of pollution was coming from non-point sources rather than being mostly from the water treatment plants. Water quality declined with flow in the fall and winter and did not meet any of the state guidelines for total N or orthophosphate in 76% and 85% of the samples, respectively.
One solution to non-point pollution is to reduce the amount of water running off urban land. A system to achieve this goal was tested in a city in southeast Spain.2 Most of the ceramic tile produced in Spain is produced in Benicàssim, so permeable pavers were constructed from tile by turning seven tile strips on their sides and bonding them together into “bricks” with spaces between the tiles. These were installed on drainage aggregate to create 200 m of sidewalks and a bike path which varied in width from 10 to 21 m. For a year after their installation, flow measurements and runoff samples were obtained before and after the permeable pavement section as well as before and after a section containing a 10m3 buried storage tank. During the monitoring period, there were 35 rainfall events ranging from 1.2–48.8 mm, with a total of 367 mm, which is about 80 mm below average. The system completely absorbed all but six events, with only 14% of the total rainfall volume being discharged, mostly during the most intense rainfall events.
Water quality was measured for three events with discharges and biological oxygen demand, coliform bacteria, and nutrients were all reduced. Heavy metal concentrations were below detection levels.