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Land Use Impacts on Water
There are two major sources of water quality impacts. The one that has
received most attention for many years is point source pollution, which
comes from wastewater treatment facilities and industrial plants. The
other major category is nonpoint source pollution, which is also known
as poison runoff. Point sources usually have a discharge permit. Your
state water quality agency can help you to understand the impacts of point
sources on your local streams.
Nonpoint sources do not have permits usually. These pollutants come from
diffuse sources, including lawns and gardens, industrial activity, parking
lots, septic tanks, agriculture, and pet droppings.
Some of the pollutants or problems that come from nonpoint sources include:
- pathogens, such as bacteria and viruses
- excess nutrients, such as nitrogen and phosphorus
- toxic contaminants, such as pesticides, lead, or PCBs
- eroded sediment, which may contain other contaminants, but is a problem even if it's not contaminated.
- Debris
As you create probable scenarios, you will want to consider the effects of both point and nonpoint sources of pollution. This section will focus primarily on evaluating how changes in land use increase or decrease nonpoint sources of pollution, and ultimately affect water quality and quantity. Let's look at how different land uses affect runoff and water quality.
Agricultural Activities
Agricultural activities are not pollution free. More nutrients (nitrogen
and phosphorus) are commonly applied than plants can use. The excess runs
off into streams or sinks into groundwater. Livestock manure alone provides
more nitrogen and phosphorus annually than is needed by all crops grown.
Fertilizer and, to a lesser extent, municipal sludge and the septage from
on-lot sewage systems add still more nitrogen and phosphorus to soils.
Much of the applied nitrogen is naturally converted to nitrates which
can travel great distances through soil and bedrock to groundwater or
to nearby surface streams. Much of the excess phosphorus can be held in
place by the soil and is therefore less mobile than nitrogen.
Pesticides and herbicides are in many instances applied at an average annual rate of 2-3 pounds per acre of cropland. Even in small concentrations pesticides and herbicides can be a public health concern. Pesticides and herbicides, like phosphorus, are less mobile than nitrogen .
Residential and Commercial Development
Many communities and industries are connected to treatment plants, in
which case their wastewater is collected and treated prior to being discharged
to nearby surface streams. However development almost inevitably leads
to increased runoff, which collects pollutants before reaching surface
streams. Some businesses and industries generate wastes which, when stored,
can leak or spill into surface streams or groundwater if they are not
controlled. Low-to-medium density development brings a proliferation of
private wells and on-lot wastewater systems which, if not controlled,
can result in health concerns regarding pathogenic microorganisms or nitrates.
Homeowners also tend to use excessive fertilizer and pesticide on lawns.
Development can harm local surface and ground water so that it cannot safely be used as drinking water. When groundwater is found at shallow depths, pollutants from the surface are not filtered out before reaching the groundwater. Pollutants reaching groundwater sources are difficult to remove and may make groundwater supplies for water supply unattractive for future water supply development.
Development can also cause problems with the quantity of surface and groundwater. When land is covered with pasture or forest, water sinks in and replenishes groundwater. Or it enters surface streams at a moderate rate so that flooding is reduced. When more land is covered with concrete and rooftops, water runs off more quickly and pollutants are not filtered out. Streams become degraded and provide poor habitat for fish and the small creatures that make up the food chain for fish. Stream banks erode more quickly, flood more often, and are shallower during dry spells.
Water trends
So how do you evaluate trends in nonpoint pollution and how land use affects
water quality? Start with the baseline information on water quality and
quantity that you collected in "Where are We Now?" Then take
a look at the trends in population and development that you have already
collected. Here are some of the questions to ask:
- Where is development expected to occur?
- What type of development is expected?
- Does your subdivision ordinance and building code require proper runoff controls?
- How much impervious cover (rooftops, concrete, etc.) is allowed-- 20%, 40%, 80%? (Studies have shown that as little as 15% impervious cover can start to affect stream quality, and 30% can have a high impact.)
- What is the cumulative effect of all development?
- How much water will be diverted out of your watershed for drinking water or because of wastewater discharges?