The preliminary information we gathered on the Silicon Valley site revealed that the site historically had a dry cleaner tenant and, before the development of the existing buildings in the late 1960s, had two gas stations located on the property during the 1940s and 1950s. This information came from a prior Phase I report provided to us when we started the project. Other corroborating sources we looked at were Sanborn Maps and old city directories. Polk's and Haines directories are the common ones we find. You can find them in your local library.
The thing about dry cleaners is that they commonly use a chemical called tetrachlorothylene, synonymously known as perchloroethylene - PCE for short, or simply "perc." PCE is a chlorinated solvent that can adversely affect health with enough exposure, typically through inhalation or direct contact in an occupational setting. PCE is also a carcinogen. Usually dry cleaner sites that have been around a number of years have caused some PCE contamination to soil and even groundwater. Once it gets into groundwater, it's a bitch to cleanup. PCE gets into the ground a number of ways, usually through the mishandling of the chemical, chemical waste, and by dumping down the drain, storm drain, or out behind the store. PCE can find its way through a crack or expansion joint in the floor slab and leach out from a sewer line. Finding a dry cleaner during the Phase I assessment is almost an automatic Phase II subsurface investigation. Or it can just kill the deal.
Gas stations store their fuel supply in underground storage tanks. These tanks and the related piping have leaked more often than not at old gas station sites. Old gas stations also may have had auto service with hydraulic hoists in the service bays. The below ground mechanisms can leak hydraulic oil. Used oil is often stored in underground tanks. Solvents may have been used to clean parts. Gas stations, as you can see, have many potential sources for contamination to get into the ground. Gasoline, diesel, oil, and, in some cases, solvents are the potential contaminants.
Compared to PCE, petroleum contamination is relatively easy to chase. It usually smells and petroleum contaminated soil is usually discolored. Petroleum products float on water such that when a release of say gasoline or diesel filters down through the soil and encounters the water table it flattens out and floats. Of course, some fraction also dissolves into the groundwater. PCE, on the other hand, is volatile and the vapors travel along preferential pathways through the soil such as sandy zones, utility trenches, and under pavement. It is not obvious by smell or color in soil at trace but still hazardous concentrations. We use a hand-held organic vapor monitor to screen soil for PCE and other solvents. PCE is also heavier than water so it sinks rather than floats in groundwater. PCE is nasty stuff.
Based on these findings, I put together a scope and budget to sample and analyze soil and groundwater samples at the former dry cleaner and two gas station locations. We will use direct-push probe methods ("Geoprobe"). Direct push probing is basically driving a hollow core barrel into the soil and retrieving a core sample of soil. When we hit groundwater, we will collect a groundwater sample using a number of methods. The most basic method is to drop a slotted 3/4 inch PVC casing down into the probe hole and pump out the sample. Another method is to drive a hydropunch to the desired sampling depth and pump out a water sample. A hydropunch is a stainless steel well screen with a retractable sleeve.
As of today, we completed two probe borings. I had selected a target depth of 20 feet, expecting we'd encounter groundwater before 20 feet. I got a call from the young geologist in the field "20 feet and still no water." I told her to go another 10 feet and call back. As it turns out we hit groundwater at 28 feet.
No signs of contamination, but we'll see what the laboratory finds in the samples. We're doing a 24-hour turnaround, which is extremely fast and pricey. But the client needs results quick so they can make a decision on purchasing the property. Real estate transactions have become fast-paced, almost too fast-paced.