How DHEC Measures Surface Water Quality

How DHEC Measures Surface Water Quality

Water quality monitoring data helps us understand why certain conditions exist in a body of water and how that water's quality may be improved.
DHEC monitors surface waters to:

  • See if they are meeting water quality standards (R.61-68)
  • Identify locations in need of extra attention
  • Determine long-term water quality trends
  • Provide background data for permitting, modeling, planning, and evaluating stream classifications (R.61-69) and standards (R.61-68
  • Help formulate permit limits for wastewater discharges with the goal of maintaining State and Federal water quality standards and criteria in the receiving streams in accordance with the goals of the Clean Water Act.

  For more detailed information, see:

    • DHEC's Organization Code for Legacy (Pre-1998) STORET data is 21SC60WQ.
    • DHEC's Organization Code for Modernized (1999 forward) STORET data is 21SC60WQ_WQX.

What Do We Measure? 
We look at a number of water quality indicators. Water quality indicators include:
Macroinvertebrates - These aquatic insects and other aquatic invertebrates associated with streams, rivers, tidal creeks, and estuaries can be useful indicators of water quality because these communities respond to integrated stresses over time that reflect fluctuating environmental conditions. The macroinvertebrate community's responses to various pollutants (i.e. organic, toxic, and sediment) may be assessed through interpretation of diversity, known organism tolerances, and in some cases, relative abundances and feeding types.

Fish Tissue - Many pollutants occur in such low concentrations in the water column that they are usually below analytical detection limits. However, over time many of these chemicals may accumulate in fish tissue to levels that are easily measured.  By analyzing fish tissue it is possible to see what pollutants may be present in waterbodies at very low levels. This information can also be used to determine if eating the fish poses undue human health concerns and to calculate safe consumption rates.

Dissolved Oxygen - Oxygen is essential for the survival and propagation of aquatic organisms. If the amount of oxygen dissolved in water falls below the minimum requirements for survival, aquatic organisms or their eggs and larvae may die. A severe example is a fish kill.  Dissolved oxygen (DO) varies greatly due to natural phenomena, resulting in daily and seasonal cycles.  Different forms of pollution also can cause declines in DO.

Changes in DO levels can result from temperature changes or the activity of plants and other organisms. The natural diurnal (daily) cycle of DO concentration is well documented.  Dissolved oxygen concentrations are generally lowest in the morning, climbing throughout the day due to photosynthesis and peaking near dusk, then steadily declining during the hours of darkness.

There is also a seasonal DO cycle in which concentrations are greater in the colder, winter months and lower in the warmer, summer months. Streamflow (in freshwater) is generally lower during the summer and fall, and greatly affects flushing, reaeration, and the extent of saltwater intrusion, all of which affect dissolved oxygen values.


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