Reading this Report About this System System Performance Design Treatment Process

System Performance

Water Quality Treatment
Data in these charts are presented as annual median event mean concentration values and median removal efficiencies. This section presents data on the stormwater treatment’s ability to reduce or remove contaminants from stormwater. The top chart represents collective water quality treatment data for two years, broken out according to season. “Summer” refers to the six-month period between May and October and “winter” refers to the six-month period between November and April for each monitoring year.

UNHSC researchers monitor specific contaminants in runoff—before it enters and after it leaves stormwater treatments—for the following contaminants:

• Total suspended solids (TSS): While there is great debate over the current methods of sampling and analyzing sediments in stormwater, TSS remains the dominant yardstick of comparison for water quality performance of stormwater treatment systems nationwide.
• Total petroleum hydrocarbons in the semi-volatile (diesel) range (TPH-D): This is the only range of hydrocarbons where the concentrations in the stormwater runoff measured at UNHSC are always well above the detection limits. Petroleum hydrocarbons are often included in regional ambient water quality criteria.
• Dissolved inorganic nitrogen (DIN): DIN includes nitrate, nitrite, and ammonia. Excessive amounts of these compounds in coastal and estuarine waters can result in harmful algal blooms and oxygen poor conditions. Nutrients like nitrogen are often included in regional ambient water quality criteria.
• Total phosphorus (TP): Excessive amounts of TP in freshwater systems can result in harmful algal blooms and oxygen poor conditions. Nutrients like phosphorus are often included in regional ambient water quality criteria.
• Total zinc (Zn): Runoff can contain a range of toxic metals from a variety of sources. Zn is the metal of highest concentration for this study area. The primary sources of Zn pollution are tire wear and galvanized metal (guard rails). Heavy metals like Zn are often included in regional ambient water quality criteria.

Water Quantity Control

This section presents data on the ability of each stormwater treatment system to reduce the flooding characteristics of runoff associated with a specific rain event. This ability is represented by measures of peak flow reduction and lag time.

“Peak flow” is the maximum rate of runoff for each rain event. The bottom graph shows the change in peak flow of runoff coming into the system (influent) and leaving the system (effluent). This observed data is then used to calculate the system’s average reduction of peak flow over time. Many communities have stormwater ordinances that require peak flow rates be reduced to a specified level.

Simply put, “lag time” is a measure of how long runoff remains within the system. Longer lag times mean that the system is reducing the “flashiness” (extreme changes in flow rate) of the runoff. This generally means that the runoff has more time to infiltrate into native soils, thus reducing total runoff and increasing the effectiveness of water quality treatment. Because the systems are all lined for research purposes, volume reduction is not considered.

Maintenance
UNHSC does not perform significant maintenance on the treatment systems at the field site as a matter of experimental design. Since these systems are often maintained minimally in practice, we want to be able to observe and chronicle if lack of maintenance contributes to a system’s failure. Our minimal maintenance activities include mowing slopes, vegetating bare spots, and removing trash. Our decision to perform minimal maintenance is related to the need to keep the systems working well enough to evaluate their performance. Based on our observations and the recommendations of stormwater manuals, we offer some—though not extensive—information on maintenance burden in this section.

Cold Climate
The performance of certain stormwater treatment system in cold climates has been the subject of much debate in the stormwater management community. This section contains observations about performance of different systems during the winters of a cold climate region.