Water Chemistry Assessment
PWD aquatic biologist downloading data from temperature datalogger
Our goal is to restore Philadelphia's streams to the point where they are safe for recreation and able to sustain healthy populations of native species. Water quality in our streams can change very quickly, so PWD uses three different assessment methods to characterize water quality and identify the causes of water quality problems.
Dry Weather Water Quality Sampling
PWD’s water quality assessment strategy has been designed to facilitate separate analyses of dry weather (i.e., baseflow) and wet weather water quality conditions. Baseflow water quality "grab" sampling yields a snapshot of water quality conditions. During initial characterization of our waterways, 39 different parameters were analyzed. After analyzing these data, we concluded that the primary pollutants of concern in most of our watersheds during dry weather are bacteria and nutrients. These parameters are now sampled quarterly during baseflow conditions at 11 USGS gage stations to track trends over time and build a long-term record of water quality changes as restoration projects are completed.
PWD Aquatic biologists collecting baseflow water quality "grab" samples
Wet Weather Water Quality Sampling
Grab sampling is effective for dry weather periods when water quality does not change very rapidly. But the influence of stormwater runoff may change water quality more quickly than PWD staff can travel to monitoring sites and collect samples. For this reason, we collect water samples from our streams in wet weather using automated samplers that begin sampling when water levels rise. Once enabled, the samplers begin collecting samples at predefined intervals according to a sampling plan. For example, samplers are programmed to collect samples more frequently at the outset of a storm event as stream discharge increases very quickly. Dozens of samples may be taken from each monitoring site over the course of a storm event in order to adequately characterize pollutant loads in stormwater runoff.
Automated sampling equipment used to collect water samples during storm events
Continuous Water Quality Monitoring
Some water quality parameters change not only due to stormwater runoff, but also due to natural cycles during the day. For example, stream temperature generally follows a pattern similar to air temperature, while dissolved oxygen and pH can change due to the influence of biological processes of photosynthesis and respiration. Monitoring these changes requires a different approach than grab sampling. Continuous water quality monitoring of Philadelphia area streams is primarily accomplished through a partnership with the USGS. Eleven USGS stream gaging stations have been fitted with continuously operating monitoring equipment that relays information about water quality to the world wide web in near-real time. USGS staff operate the stream gaging equipment while PWD staff are responsible for maintaining the water quality sensors.
More information about this program is available on the PWD/USGS Cooperative Philadelphia Water Resources Monitoring Program Website
PWD aquatic biologists published an article about PWD's water quality monitoring program in Water Environment & Technology, a monthly publication of the Water Environment Federation. Download article 2.3MB PDF.