Basics of Environmental Sampling and Analysis - Fundamentals of Environmental Sampling and Analysis - page 30

The analysis of specific spatial or time series data is a highly specialized area in
statistics. Its theories are not even covered in standard statistics texts and are beyond
the reach of most environmental professionals. Fortunately, there are particular
computer programs that are manageable for the general reader. One example of such
useful software is the Geostatistical Environmental Assessment Software (Geo-EAS)
that was developed for performing two-dimensional analyses of spatially distributed
data (downloadable from www.epa.gov/ada/csmos/models/geoeas.html). One of its
unique features is the ability to plot contour maps of pollutant concentration or other
variables. Examples of applications include Pb and Cd concentrations in soils
surrounding a smelter site, outdoor atmospheric NO 2 concentrations in metropolitan
areas, and regional sulfate deposition in rainfall (EPA, 1991). Another software
package, called ANNIE, was developed by the USGS for the analysis of time-series
data (USGS, 1995).

2.3 ESSENTIAL HYDROLOGY AND GEOLOGY

2.3.1 Stream Water Flow and Measurement

Measurement of stream flow (i.e., discharge in cubic feet per second, or cfs) is
important for surface water quality monitoring. In many large waterways in the US,
flow data, gauge height, and discharge, can be obtained in almost real time from the
USGS Web site (http://water.usgs.gov/index.html). If the sampling site is close to the
USGS gauging station, field personnel can refer to the gauge height during sampling.
The gauge height (ft) is converted to stream flow (cfs) by a rating curve that is
calibrated by the USGS for that particular station.
When a stream flow needs to be determined, it is often conducted prior to the
collection of chemical and biological water samples. There are several acceptable
methods for flow measurement, but a flow meter is most commonly used. The flow
meter measures the water flow velocity (V), in feet per second, which is then
converted to the discharge by using the cross-sectional area of the stream (A). A
variety of flow meters are available; some are based on electromagnetism (e.g.,
Marsh-McBirney, Montedoro-Whitney) and others are propeller type (Price Pigmy).
Since velocities vary both horizontally and vertically across a stream cross-
section (velocity is slower toward the bottom and near the bank), multiple
measurements must be made to determine the average flow:

Q ¼ðW 1 D 1 V 1 ÞþðW 2 D 2 V 2 Þþ...ðW n D n V n Þ ð2 : 25Þ

where W and D denote width and depth, respectively. The number (n) of flow cross
sections depends on the stream width (use 0.5 ft if total width is less than 5 ft; use
minimum of n ¼ 10 and n ¼ 20-30 for a wider stream). For the same cross section,
measure the velocity twice, once at 20% and once at 80% of the water depth if water
is deeper than 2.5 ft, or measure once only at 60% if water depth is less than 2.5 ft
(TNRCC, 1999). The average velocity is at 6/10th of the total water depth from the
water surface.