Environmental Sampling Design - Fundamentals of Environmental Sampling and Analysis - page 67

(c) Compare the results from (a) and (b) and comment the difference.

19. A stratified random sampling plan was adopted for a contaminated site as a result of a
recent oil spill in an open agricultural land. Three strata were chosen as shown in the
diagram below: (1) the heavily contaminated surface soil (0-2 ft); (2) the unsaturated
soil (2 ft to the groundwater level at 7 ft deep); and (3) the saturated zone (7-27 ft). The
results of a probe chemical (benzene) are shown in the table below. If one of the
objectives was to estimate the degree of contamination in the entire aquifer (0-27 ft):

(a) Estimate the overall mean, standard deviation, and confidence interval (at 80%
confidence level) using proportional allocation method (i.e., based on the depth or
volume ratio).
(b) Estimate the overall mean, standard deviation, and confidence interval (at 80%
confidence level) using optimal allocation method (i.e., based on the variation of
each stratum).

Number of Benzene

Stratum Depth (ft) samples taken concentration (mg/kg)

Surface soil (0-2 ft) 2 15 x 1 ¼ 25; s 1 ¼ 4:0

Unsaturated soil (2-7 ft) 5 6 x 2 ¼ 17; s 2 ¼ 2:0

Saturated soil (7-27 ft) 20 5 x 3 ¼ 10; s 3 ¼ 1:0

Oil spill site

Surface soil (2 ft)

Unsaturated soil (5 ft)

Groundwater table

Saturated soil (20 ft)

Groundwater flow

Aquiclude Aquiclude

20. A field scale remediation demonstration project was studied to test the efficiency of
soil vapor extraction to remove BTEX compounds from contaminated soils in the
unsaturated zone due to a rupture of chemical container. It was estimated that the
contaminated area should be within the boundary of 100 m (north-south) 150 m
(east-west). After the demonstration project was completed, a detailed sampling plan
needs to be executed to precisely map the spatial pattern of BTEX concentrations.
Budgetary constrains allows for 50 samples to be collected for BTEX analysis of soil
cores. (a) If systematic grid sampling was employed, determine the grid spacing (L)
using square grid? (b) Draw a schematic diagram showing how these 50 sampling
points are located?