Ask The Science Guy!

How Land Use Affects Water Purity
By Paul Susca

There are lots of good reasons to conserve land – saving places for birds and wildlife to live, saving places for our children and ourselves to explore and enjoy, preserving the rural character of communities, ensuring that special places will remain special. Another great reason is to protect our rivers, streams, lakes, and ponds. That’s because a water body is the product of its watershed – the land that ultimately drains to that water body.

How does that work? Simply put, when it rains, the watershed takes a bath. If a watershed is covered with roads, rooftops, fertilized lawns, parking lots, row crops, or any land that has been disturbed or isn’t covered with natural vegetation, that land is bathed by the rain, and the dirty bathwater flows to streams, rivers, lakes, and ponds. The “bathwater” is called polluted runoff, stormwater, or non-point pollution.

On the other hand, when rain or snow falls (or melts) on undeveloped land, about 50 percent of the water (over the course of the year) soaks into the ground and replenishes the groundwater that feeds streams and water supply wells year-round, about 10 percent runs off along the surface, and the rest returns to the atmosphere. The 10 percent that runs off the land surface is typically clean, since plants and natural plant litter slow down the runoff and filter out any eroded soil.

A vegetated buffer of a few hundred feet can remove 80 percent of sediment and pollutants carried by runoff from developed areas such as roads and parking lots. That’s why state and local shoreland protection laws are important tools in preventing pollution; they aim to maintain vegetated buffers along rivers and lakes and otherwise limit land disturbance near shore. The width of a buffer is not the only thing that affects its filtration ability – the slope of the land and the type of vegetation matter too – but you have to start somewhere.

From a water drop’s perspective, land in its natural state does more than filter runoff. When the natural land cover is removed, the percentage of rain that soaks in is reduced from 50 percent to as little as 15 percent in highly urbanized areas, and the percentage that runs off increases to as much as 55 percent. Even if the runoff is clean – which it seldom is – the changes that take place in the water balance have profound effects on streams and rivers.

When as little as 10 percent of a watershed is paved or covered with other hard surfaces such as rooftops or compacted soil, a shift takes place. During dry periods there is less streamflow because groundwater is not being replenished as much, and that hurts aquatic life. When it does rain, streamflows are faster and higher because of the increased runoff. Extremes in flow – both low and high – create a wider streambed with less water during dry periods and also change the makeup of stream-bottom gravels, sands, and rocks that normally provide a variety of habitat for a variety of aquatic life.

Development doesn’t have to mean more water pollution. A variety of low-impact development techniques – including stream buffers and new approaches to stormwater management – have proved their effectiveness in protecting water bodies. But nothing works as well as land conservation in protecting our water and so many other things we value.