# EPA On-line Tools for Site Assessment Calculation

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#### Background

The hydraulic gradient, along with the hydraulic conductivity, determines the ground water flow rate. On-Site provides three methods for determining the gradient:

• Two points and a linear gradient
• Three points, linear gradient, and direction
• Multiple flow line segements with linear gradients

#### Why include these choices?

The gradient is calculated from

where i is the gradient, h is the hydraulic head at two points 1 and 2 and x is the location of points 1 and 2. A linear relationship between the heads at points 1 and 2 is strictly correct only for confined flows. In inconfined flow, the shape of the water table is curved (see Bear, 1972 for example). So at a site we might approximate the gradient as a line between the two points, as implied by the equation. The accuracy of this approximation depends on how curved the water surface is. Generally, as a discharge point is approached the slope of the water table steepens and the linear approximation is less accurate. For these cases using multiple gradients over the length of the flow path may may sense. Thus the "Multiple Segement" gradient calculator provides an explicit reminder that the gradient may not be uniform across a site. Only a single "segment" version of the "Three Point" gradient calculator is supplied. Multiple segements can be addressed by breaking up the flow domain into triangles.

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