Using the following data estimate the vertical velocity and

Using the following data, estimate the vertical velocity and recharge rate. The samples were collected from a well nest in a sandstone aquifer. The mean annual air temperature in the recharge area is 9° C and the elevation is 800 m. The samples were collected in 2000.

Screen Depth Below Water Table (m) 5.1 7.9 12.6 18.7 CFC-11 pg/L. 471 325 22 CFC-12 (pg 244 200 158 10

Solution

Recharge rates in unconfined aquifers were calculated from groundwater-age and depth data in two types of well networks: multi-level (nested) wells and wells screened near the water table. In most cases, nested wells in a given aquifer were aligned along the same or similar flow paths, whereas the water-table wells were randomly located. Recharge rates for the nested wells were calculated using the approach of Vogel which assumes a logarithmic age-depth distribution.

R = (n-Z/t) – ln[(Z/Z-z)]

Where R is recharge rate, n is porosity, Z is thickness of the aquifer saturated zone, t is the tracer-based groundwater age of a sample, and z is depth of the sample.

All of the selected nested-well networks had sufficient information on well location within the flow system to select only those wells located in recharge areas. This approach for calculating recharge is subject to several assumptions, including that the unconfined aquifers received distributed recharge and have constant porosity and thickness, and that individual samples have discrete ages. Mean age or residence time (t) of groundwater in the sampled flow systems is given by:

T = n – Z/R

Therefore the vertical velocity = 0.002 m/ day , recharge rate = 136,167 m³ / day.