The Groundwater Gradient Factor in Phase I Environmental Site Assessments
Gradient is a degree of inclination like that of a river bed, usually described as the number of feet the incline drops per mile. In groundwater, the gradient is the number of feet the water table drops per mile.
| On the surface, water always flows downhill. |
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And so it does underground.
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Or is it really so?
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.
Thus, in Phase One Environmental Site Assessment we search for properties that are located upgradient to see if they have contaminated the groundwater. In this search, however, we do not ignore properties downgradient from or crossgradient (sideways) to our property, especially if they are nearby.
The reason is that within narrow zones, groundwater flow directions may vary from the general area.
In Los Angeles Basin, for example, groundwater generally moves south, southeast and southwest.
However, it is not uncommon to find places where groundwater goes in opposite directions. There are several factors that can divert the local flow of groundwater. |
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You could have a dewatering operation happening inside a construction excavation:
You may have a heavy pumping in the neighborhood:
There could be subterranean seismic faults:
In one case we had the retaining walls of a drainage channel diverting the groundwater flow in the neighborhood from southeast to northwest:
Therefore, when we identify sources of groundwater contamination in the neighborhood and when groundwater is shallow, we often express concerns for migration of the contamination into the property in question, even if the sources are considered to be downgradient or cross gradient when looking only at the general aspect of the area.
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