WATER QUALITY - Water Quality Engineering in Natural Systems - page 7

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WATER QUALITY

2.1 INTRODUCTION measured relative to either the water-quality criteria or

the water-quality standards associated with their desig-
nated use. The acceptable water quality for a natural water body

generally depends on its present and future most benefi-
cial use. Commonly designated beneficial uses include
public water supply, recreational use, fisheries and shell-
fish production, agricultural and industrial water supply,
aquatic life, and navigation. Each of these designated
uses has its own set of water-quality criteria , which
includes the physical, chemical, and biological attributes
that are consistent with the designated use of the
water body. Water-quality criteria generally take into
consideration both human health and aquatic life
impacts. Human-health based water-quality criteria
are derived from assumptions related to the degree of
human contact, quantity of water ingested during human
contact, and the amount of aquatic organisms (e.g., fish)
consumed that are derived from the water body. Aquatic-
life water-quality criteria are derived from mortality
studies of selected organisms exposed to various levels
of contamination in the water, as well as other factors
that measure the health of aquatic ecosystems. Overall,
water-quality criteria are formulated to maintain the
physical, chemical, and biological integrity of a water
body, with alterations in the physical and/or chemical
condition generally resulting in changes in biological
condition.
By definition, water-quality criteria are not legally
binding or enforceable; however, when they are included
as regulatory requirements (which are legally enforce-
able), they are typically referred to as water-quality stan-
dards . The quality of natural waters should generally be

2.2 PHYSICAL MEASURES

Physical measures that directly affect the quality of
aquatic life habitat include flow conditions, substrate,
in-stream habitat, riparian habitat, and thermal condi-
tion. These measures are described below.

2.2.1 Flow Conditions

Slope and velocity divide streams into four categories:
mountain streams, piedmont streams, valley streams,
and plains and coastal streams. Mountain streams , which
are sometimes called trout streams , have steep gradients
and rapid currents; streambeds consisting of rock, boul-
ders, and sometimes sand and gravel; and are well
aerated and cool, with temperatures rarely exceeding
20°C. Piedmont streams are larger than mountain
streams, with depths up to 2 m (6 ft); have rapid cur-
rents with alternating riffles (shallow, fast-moving
waters) and pools (deep slow-moving waters); and
streambeds typically consist of gravel. A typical pool
and a typical riffle in Elizabeth Brook (Massachusetts)
are shown in Figure 2.1.
Valley streams have moderate gradient and current
with alternating rapids and more extensive quiet waters
than in piedmont streams. Plains and coastal streams are
typically the lower elevation stretches of rivers and