WATER QUALITY CRITERIA
WHEN DO
POINT SOURCES CAUSE THE GREATEST WATER QUALITY PROBLEMS?
WHY?
Point source receiving water standards based on low flow conditions, i.e. the 7-day, 10-year low flow. This is logical for point sources since discharges (other than diffuse point sources - feedlots, mine land, urban runoff) are relatively constant and would be expected to have the greatest impact on stream water quality during low flow conditions because of reduced dilution.
WHEN DO
DIFFUSE SOURCES CAUSE THE GREATEST WATER QUALITY PROBLEMS? WHY?
Conditions are generally exactly the opposite with NPS
pollution. Since most NPS water pollution occurs during larger storms, NPS pollution will generally be worst
during extreme runoff producing events.
Consider the case of the Chesapeake Bay: NPS pollution sources predominate during the winter and spring when there is a lot of runoff. Point sources predominate during the drier summer months. Water quality problems (eutrophication) are generally the most severe during the summer. What does this mean in terms of pollution control planning? Which is more important, point or NPS in this case?
WHAT
IS WASTE ASSIMILATIVE CAPACITY AND WHY IS IT IMPORTANT?
HOW
DOES WASTE ASSIMILATIVE CAPACITY AFFECT THE SETTING OF RECEIVING WATER
STANDARDS?
Waste Assimilative Capacity is the natural ability of a water to accept potential pollutants without harmful effects. The concept of waste assimilative capacity is very important because many substances are not pollutants at low concentrations and in fact, may be essential for life. Also, it is not economically feasible to remove all pollutants from water. Therefore, we must make use of the natural waste assimilative capacity of water. The alternative is zero discharge. Waste assimilative capacity varies for different substances: Easily degradable organics, microorganisms, and nutrients have relatively high assimilative capacity. Substances with low assimilative capacity include: toxic organic compounds (PCBs, DDT, Kepone, chlordane, etc.) and heavy metals.
Stream Standards:
Water quality criteria are often based on stream water quality. For example, water bodies in a state may be divided into four or five different categories depending on their natural quality and intended use. Water quality standards are then established with these conditions in mind (Water Quality Based Effluent Limitations, WQBELs). Naturally poorer quality streams may have lower water quality criteria while pristine water bodies may have high standards.
SPECIFIC WATER QUALITY PROBLEMS
WHAT
ARE THE ADVERSE EFFECTS OF EXCESSIVE SEDIMENT IN WATER?
Most significant pollutant in terms of mass loading. Negatively affects water quality by:
1. Increasing
the turbidity of water which reduces
light penetration and thus photosynthesis.
2. Destroying the habitats of benthic organisms by covering them with sediment.
3. Interfering with respiration of fish and other aquatic organisms by clogging or irritating their gills, etc.
4. Transporting pollutants such as bacteria, nutrients, metals, and toxic organics to receiving waters.
5. Increasing water treatment costs (suspended solids removal required). Water treatment plant residuals are not regulated the same as wastewater residuals.
6. Reducing the aesthetic value of water for recreational use.
HOW
CAN SEDIMENT IN RECEIVING WATERS BE CONTROLLED?
Sediment problems are controlled by removing sediment from water via settling (with or without flocculating chemicals), filtration or by preventing sediment from entering the water in the first place. This latter approach is the most cost effective approach for NPS pollution. No specific standards or criteria exist for TSS.
WHAT
WATER QUALITY PARAMETER IS THE BEST INDICATOR OF THE
SUITABILITY OF WATER FOR AQUATIC LIFE?
WHY?
WHAT
ARE DESIRABLE D.O. CONCENTRATIONS?
Dissolved oxygen is
the primary parameter on which the suitability of water for aquatic life
is determined. It is also the major parameter used in most waste assimilative capacity studies since oxygen is required for the assimilation of organic wastes.
Dissolved Oxygen Receiving Water Standards, mg/L Salmonids Non-salmonids
30-day mean 6.5 5.5
7-day mean minimum 5.0 4.0
1-day mean minimum 4.0 3.0
WHAT
ARE SINKS AND SOURCES OF D.O. IN SURFACE WATERS? IF LOW D.O. IS A WATER QUALITY
PROBLEM, WHAT CAN BE DONE TO RAISE IT?
1. Atmospheric re-aeration. Increases as the turbulence of water increases.
2. Production of oxygen by photosynthetic aquatic plants.
1. Deoxygenation of biodegradable organics by bacteria and fungi.
2. Benthic BOD. Similar to deoxygenation by bacteria and fungi except it occurs in benthic sediment or when benthic sediment is resuspended during storms.
3. Nitrification, in which oxygen is used by bacteria to oxidize ammonia and organic-N to nitrates.
4. Respiration by aquatic animals and respiration by algae and aquatic plants during the night.
BOD is an important parameter used in DO studies. BOD is a measure of the amount of biodegradable organic matter in water.
Example
1
If
1 L of water with a BOD of 10 mg/L and DO concentration of 4 mg/L was added to
1 L of water with no BOD and a DO content of 8 mg/L, determine the eventual DO
of the resulting mixture, assuming their was no reaeration.
Initial BOD = (1 L)(10 mg/L) + (1 L)(0 mg/L) = 10 mg BOD
Initial DO = (1 L)(4 mg/L) + (1 L)(8 mg/L) = 12 mg DO
Final DO = Initial DO - Initial BOD =
12 - 10 = 2 mg DO
= 2 mg DO /
2 L water = 1 mg/L DO
Actual DO and
BOD fluctuations in streams are more complex than this and are a function of
initial DO and BOD, BOD removal coefficients, reaeration
coefficients, BOD deoxygenation coefficients, benthic
oxygen demand, water temperature, water velocity and turbulence, water depth,
nitrification, photosynthesis, etc.
No specific standard but the nearer saturation the better. Supersaturation can be harmful to fish.
CATEGORIZATION
OF NPS POLLUTANTS
Phosphorus, heavy metals, chlorinated hydrocarbons (DDT), organics, PCBs, organic-N, microorganisms, etc. Most if not all predominately sediment-bound pollutants exist in a chemical equilibrium with their dissolved phase. The concentration in solution can usually be expressed as a function of the pH, temperature, concentration in the sediment-bound or solid phase, and other factors.
SUPPOSE
THAT PCB IS A WATER QUALITY PROBLEM IN A WATERSHED WHERE PCB LADEN OIL
CONTAMINATES THE FIRST FEW CM OF SOILS IN THE WATERSHED. KNOWING THAT PCB IS
TRANSPORTED PRIMARILY IN A SEDIMENT-BOUND FORM, WHAT APPROACHES COULD BE TAKEN
TO MINIMIZE LOSSES TO RECEIVING WATERS?
In agriculture, wind and water erosion are the source of
sediment-bound pollutants. Since wind
and water interact almost exclusively with the soil surface, only the top cm or
so of the soil surface is important in sediment-bound pollutant loss. This is
the area where wind, surface runoff, and raindrops have an opportunity to mix
with the soil and detach and transport soil particles or dissolve soluble
pollutants. Since the upper few mm are the source of sediment-bound pollutants,
this is where we must concentrate our pollution control efforts, i.e. change landuse and land management practices.
Nitrate, dissolved organic-N, dissolved organic-P, many herbicides and insecticides, "petroleum products", salts. Important to realize that dissolved pollutants exist in equilibrium with their solid or sediment-bound phases. So, removal of the dissolved phase pollutant may result in release of adsorbed pollutant from the sediment until the solid-dissolved phase equilibrium is reestablished. Similarly, if all contaminated sediment is removed, dissolved pollutants may come out of solution and recontaminate the remaining sediment.
Nitrogen is a good example of how complex the dissolved and solid phase interactions can be. Agriculture is also the source of the majority of N in the environment so N control efforts must address agriculture.
Dissolved forms of N include: NO3 , NO2 , NH4 , NH3 and organic forms. The biggest DIFFUSE problem is with NO3 (nitrate) because it moves with water and is not readily adsorbed to soil particles. Researchers estimate that 10 to 50% of NO3 loss from cropland is due to subsurface leaching. This percentage is even greater in tile-drained areas. Losses of total-N are much greater in surface runoff than subsurface flow because the major N form in soil is organic-N (90%+), which is predominately sediment-bound. This organic-N is slowly mineralized to mineral, plant available forms (NO3 and NH4) as the mineral forms are used by plants or leached. The plant available portion of N in the soil is only 1 to 3% of the total-N in the root zone.
WHAT
IS CONSERVATION TILLAGE? EXAMPLES? GOOD WITH ALL
CROPS?
PROBLEMS?
With conventional tillage (less than 30% surface residue after planting, leads to increased erosion and surface runoff) more than 75% of N is lost in surface runoff. With conservation tillage (greater than 30% of soil surface covered with crop residue after planting) subsurface losses may be greater than surface losses because erosion and surface runoff decrease relative to conventional tillage and infiltration and percolation tend to increase. There is considerable uncertainty and disagreement as to how significant this effect is.
WHY
DO PESTICIDE PROBLEMS SEEM TO BE DECREASING AS COMPARED TO IN THE SIXTIES? WHO
RAISED THE ALARM ABOUT PESTICIDES IN THE SIXTIES?
PESTICIDES: What are safe concentrations? We don't really know because only a small fraction of the pesticides in use have been thoroughly investigated. Pesticides don't appear to be as big a problem as in the past because
1. New insecticides are much less persistent than their predecessors (i.e.. replacement of chlorinated hydrocarbons such as DDT with organic phosphorus compounds).
2. Newer insecticides, though much shorter lived, are orders of magnitude more toxic than their predecessors. Consequently, misapplication results in immediate consequences. This has facilitated enforcement of environmental regulations. Since the newer compounds are more toxic, their use on a mass basis is declining significantly.
3. Herbicide use is increasing significantly due to growth of no-till and conservation tillage. Fortunately, herbicides are designed to interfere with plant growth and not vertebrates. Consequently, they are not very toxic to animals. Herbicide use has actually decreased during the mid to late 80's in the US. Presumably due to poor farm economic conditions.
4. Most problems with pesticides can be attributed to misapplication and/or abuse. Usually, the applicator (farmer or home owner) is the first affected.
5. Pesticides are so expensive that farmers cannot afford to over apply them. People in urban areas are the worst at over application.