Example applications of this tool can be viewed in this document File:Plume examples 2. The following examples utilize the above guidance, including the screening assessment see File:Contamination screening checklist for stormwater infiltration. A bioinfiltration basin rain garden is proposed on city land currently used for recreation. Soils in the area of the proposed BMP are fine sands and the aquifer material is coarse sand, giving an Influence zone distance of 75 feet. The surrounding land use within 75 feet of the proposed BMP is single-family residential and a college.
Interviews with the property owner indicate the site is not contaminated and has no history of contamination. There are no observed potential sources of contamination on the property. On an adjacent property a small quantity hazardous waste generator was identified using What's in My Neighborhood. Interviews with the owner of the small quantity hazardous waste generator indicates there is no history of contamination and the potential for contamination is very low. A site visit indicates there are no identified sources of contamination within 75 feet of the proposed BMP.
Using the screening checklist, infiltration is deemed appropriate for this site. To see the checklist for this site, follow this link File:Contamination screening checklist for stormwater infiltration.
An existing parking lot will be replaced by a permeable pavement parking lot. Land use at the site of the proposed BMP is commercial business. The underlying soil is fine sand and the aquifer material is gravel, giving an Influence zone distance of feet. Adjacent land uses within feet of the proposed BMP include multiple-family residential, commercial, and industrial. There are also high volume transportation corridors within feet of the proposed BMP.
Interviews with the property owner indicate there is no known contamination on the site. Using What's in My Neighborhood , an old tank leak site is identified adjacent to the property. Clicking on the tank site in What's in my Neighborhood and opening the document associated with the site indicates a closure letter was issued by the MPCA 20 years ago indicating the site has been adequately addressed.
The next step is to conduct a site visit. The visit reveals no evidence for contamination. However, industries north of the property are within feet of the proposed BMP. Loading docks are present at one of the industrial facilities. Loading docks represent a potential stormwater hotspot. Using the guidance on potential stormwater hotspots it is determined there are no activities that represent a contamination risk. It is therefore determined that infiltration is appropriate at this site.
Note that we could have conducted a mounding analysis for the BMP to determine if adequate separation could be achieved from the loading dock. This would assume the loading dock was a source of contamination. As part of a road reconstruction project, a swale with check dams will be constructed on permeable soils, thus allowing for infiltration.
The underlying aquifer material is unknown, giving an Influence zone distance of feet. In addition to the road, which is a major transportation route, land use within feet of the swale consists of low density residential, a marina, and a railroad corridor. The site does not have known or past contamination. A site visit indicates there are fuel tanks at the marina and extensive soil staining along the railroad corridor.
The fuel tanks are located feet from the swale and, at their closest point, the stained soils are within 75 feet of the swale. Since the railroad has been in operation for several decades and because many different potential sources exist for the stained soils, it is difficult to obtain accurate information about the stained soils without field sampling.
Rather than conduct soil sampling we assume the stained soils represent contaminated soil. Using the County Geologic Atlas for the area, groundwater flow is determined to be toward the river.
A mounding analysis is conducted using the USGS mounding calculator with the following inputs. Using the mounding calculator, we are looking for the distance at which the mounded elevation is 0. This is the height considered to be inconsequential for impacting groundwater flow. Using the information above, that distance is feet. Since this is the distance from the center of the swale and we want the distance from the edge of the swale, we subtract 10 feet off this value, giving a distance of feet.
We multiply this by the safety factor of 2 to give a distance of feet. This is greater than the distance to the stained soils 75 feet so we choose not to infiltrate at this site because of the potential for mobilizing contaminants. Note that contamination has not been confirmed through sampling.
If the owner of the BMP wanted to continue pursuing infiltration at this site, a Phase 1 or Phase 2 investigation is highly recommended. Additional investigation would focus on soil and groundwater sampling in the railroad corridor. Alternatively, groundwater modeling could be conducted to determine if infiltrated stormwater would impact groundwater beneath the railroad corridor.
An infiltration BMP is proposed for a site located in a busy commercial area with major transportation corridors. The BMP is proposed for the northeast corner of the site, where there is no history of contamination. However, the western portion of the site is contaminated and has not been remediated. It is therefore appropriate to not infiltrate at this site. If the owner of the BMP wants to continue pursuing infiltration at this site, a Phase 1 or Phase 2 investigation is highly recommended. This site poses significant risk of mobilizing contamination.
In addition to existing known contamination, the nature of the area suggests a high potential for contaminated soils and groundwater. This includes numerous adjacent sites that have a history of contamination. There are also several features on the site that warrant concern, including considerable debris, monitoring wells, stained soils, odors, ponded water, and soil stockpiles. The screening approach does not verify the presence of contamination and therefore represents a conservative approach to infiltrating stormwater.
The following discussion is aimed at individuals or entities who want to conduct additional site investigation to determine if infiltration is feasible at a site. It is recommended that a Phase 1 or Phase 2 environmental site assessment be conducted at this point. The above screening assessment provides a qualitative analysis and identifies sites where a reasonable risk of mobilizing contaminants exists. There are sites that fail this screening where infiltration can still occur without mobilizing contamination.
However, you must be aware of the risks associated with infiltrating at these sites.
If you choose to continue pursuing infiltration at this point, a Phase 1 ESA is recommended. A qualified consultant should undertake and complete the investigation.
source url Examples of qualified individuals include the following:. If there is any reason to suspect soil or groundwater may be contaminated at a site, or if the current or recent land use is associated with activities involving potential contaminants, a Phase 1 ESA should be completed by a professional consultant. Industrial areas and areas where gas stations or dry cleaners are present are examples of areas where a Phase 1 ESA is recommended.
A Phase I ESA involves collecting information similar to that for conducting a screening assessment , which is discussed in detail above. During a Phase II Site Assessment, surface soil, subsurface soil, and groundwater samples are collected for laboratory analysis. This section provides a summary and step-by-step process for conducting a Phase 2 ESA. Links to detailed information are included. A workplan should be prepared by a qualified professional.
The workplan should include a detailed description of sampling methods, sampling equipment, sample analysis including laboratory and field analysis , quality assurance procedures, a site safety and contingency plan, and a schedule. Because the purpose of this investigation is to determine if stormwater infiltration is feasible, the Phase 2 investigation only includes soil and groundwater sampling.
However, if contamination is found at the site, other regulatory requirements may apply. Identifying potential contaminants of concern COC can save considerable cost associated with laboratory analysis.