Groundwater Gradient and Phase I Environmental Site Assessment
Environmental Enlightenment #312
Gradient is the inclination of a riverbed, usually described as the number of feet the river drops per mile. Water flows downhill.
Take a close look at the water in the creek as it bounces from rock to rock on the way down and you’ll see eddy currents going against the flow in various directions. And so it does underground.
In the investigation of properties, we are interested in potential impacts from the neighbors: contaminated groundwater from an uphill (upgradient) source can migrate downhill (downgradient) into our property of concern.
The diagram below describes a case where a plume is migrating downgradient across two city blocks.
Thus, in Phase One Environmental Site Assessment we search for properties that are located upgradient (uphill) to see if they have contaminated the groundwater. In this search, however, we do not ignore properties downgradient (downhill) from or cross-gradient (sideways) to our property, especially if they are nearby.
The reason is that within narrow zones, groundwater flow directions may vary from the general flow in the area.
There could be subterranean seismic faults:
In one case we had the retaining walls of a drainage channel reversing groundwater flow 180 degrees.
Therefore, when we identify sources that have contaminated groundwater in proximity to a property of interest, we may express concerns for migration of the contamination into the property, even if the sources are downgradient (downhill) or cross-gradient (sideways) when looking at the general flow of groundwater in the area.