Stormwater and groundwater will be collected during the wet season and dry season for two years and monitored for a variety of contaminants. The proposed monitoring efforts include examining the following:
- A difference in stormwater quality before and after runoff has passed through the vegetated and structural pre-treatment features to evaluate the effectiveness of pre-treatment.
- Differences between contaminant concentrations in stormwater just prior to entering the dry well will be compared to water collected in the vadose zone and at the water table to evaluate potential pollutants introduced through the dry well.
- Differences in groundwater quality upgradient and downgradient of the dry wells. This data will help to interpret the source of contaminants in groundwater, should any be detected.
Contaminants of Interest
A wide range of urban contaminants will be evaluated in both stormwater and groundwater. The list of analytes includes:
- Drinking water metals
- Organics (volatile and semi-volatile)
- Pyrogrenic polycyclic aromatic hydrocarbons
- Pyrethroid pesticides
- Total petroleum hydrocarbons
- General physical and minerals
Stormwater samples will be collected from the first flush event and from two additional consecutive rain events that produce runoff volumes of a minimum of ½ ” within 24-hour period, for two consecutive years (total of six sampling events). Samples collection will begin within 2-hours of the initiation of the rain event and continue for the entire event. At each event, a flow-weighted composite sample of approximately 10 liters of stormwater will be collected from the pipe that conveys water from the sedimentation well to the dry well. This sample will be analyzed for the full suite of contaminants. In addition, a grab sample will also be collected from the same location for measurement of conventional parameters (i.e., pH, DO, turbidity (NTU) and for a laboratory analysis of total suspended solids). An additional grab sample will be collected as stormwater enters the vegetated pre-treatment feature for the measurement in the lab of total suspended solids and pyrethroids. Further details of the sampling methods are included in the project’s Quality Assurance Project Plan, Appendix 1 – Standard Operating Procedures. The diagram below illustrates where the composite and grab samples will be collected.Dry wells system to left with stormwater sampling locations.
Groundwater samples will be collected from three groundwater monitoring wells and a vadose zone well at each site for the same three rain events described above for two years. Groundwater levels will be regularly monitored using a combination of manual and continuous measurements. During the rainy season, samples will be collected approximately one week after each storm event to allow time for water to infiltrate through the vadose zone. The exact timing of the delayed collection will be determined by monitoring well water level after rain events.
In addition, a fourth sample will be collected during the dry season from the three monitoring wells. It is unlikely that there will be sufficient water at the level of the vadose zone well to collect a sample during the dry season. Approximately 10 liters of groundwater will be collected for analysis of the contaminants listed above.
For more details on the project’s stormwater and groundwater monitoring, click on icon below to review the project’s Quality Assurance Project Plan (QAPP)
Quality Assurance Project Plan (QAPP)
This report summarizes the standard operating procedures for monitoring stormwater and groundwater.