Wow, where to begin. This is not an uncommon problem, especially in areas where groundwater is shallow.
One of my sites was a bulk petroleum distribution site, with another bulk petroleum distribution site across the street to the south. Groundwater was shallow (~5-8 feet with seasonal variability) with flow to the east toward a seasonal canal. Betweeen the two sites in the center of the road, there was a deeper (10~12') sewer lateral in permeable backfill that sloped to the east and acted as a drain on the site, affecting both groundwater levels and contaminant transport. By the time the problem was fully uncovered, MTBE had travelled east in the utility corridor to the canal, which acted as a shallow barrier to fluid migration, and spread laterally (to the north and south) almost 1/2 mile in each direction. BIG PROBLEM!!!
In general, utility trenches are more permeable than surrounding soils, and they may serve as collection sumps for surface runoff/infiltration of water and contaminants, even where the groundwater table is deeper than the trenches. Becasue of their higher permeability, any material that enters the trenches can travel some distance before fully percolating into underlying soils.
One approach for your research is to talk to municipal city workers - they will know which trenches are good and bad. I have often heard things like "Oh, yeah, everyone knows that stretch of trench is contaminated.....it stinks so bad I get a headache everytime I go in there." Whenever I think utility corridors may have an impact on my site, I consult workers ASAP.
