More systems, more nutrients


April 10, 1993|By TOM HORTON

Just east of the little creek where I live is the town of Salisbury, and the sewage treatment plant there spends considerable money complying with tough standards on discharges to Chesapeake Bay.

To the west of us are mostly farms -- grain and poultry operations -- and the pressure is stepping up smartly on the farmers to keep their manure from running into the bay.

But we suburbanites, who increasingly live and flush between sewer and farm, are most likely not carrying our share of the bay cleanup burden.

We are perhaps more ignorant than hypocritical in our situation, which involves merely having bought homes hooked to septic tanks, one of the least-addressed sources of bay pollution.

And we along the creek are certainly not alone. An estimated one-third of all homes in the bay's six-state watershed flush their toilets into septic tanks.

A septic system is basically a concrete tank buried in the yard, collecting wastewater through pipes extending from the home. The solids settle to the bottom and are gradually liquefied by bacteria.

Liquid waste flows from the top part of the tank into a drain field, which consists of perforated pipe buried in trenches filled with porous sand or gravel. As wastewater spreads through the soils, bacteria and viruses harmful to human health are filtered and rendered harmless.

Properly installed, septic tanks do their job out of sight, out of mind for years at a time. Many people aren't even sure they've got one; but if you don't pay a regular sewer bill, chances are you do.

Ironically, the better septic tanks do their job of dispersing wastewater through the earth, the more they pose a knotty problem for the bay.

This is because they also inject the nutrients nitrogen and phosphorus into the soil, and into the ground water that oozes into waterways through their bottoms and banks.

For the most part, nitrogen in the bay region's ground water poses no threat to human health, and septic tanks were never designed to remove it. But excessive nitrogen is one of the most critical pollutants in the bay's current decline, causing problems ranging from the death of underwater grass bed habitats to the depletion of oxygen in the water.

Maryland officials estimate that of 20 million pounds of nitrogen contaminating the state's ground water each year, nearly 30 percent may come from septic systems. But this and other estimates only sketch the potential of the problem. No one has much idea how much nitrogen from septic systems reaches the bay.

The few studies of septic systems and nitrogen show wide variances in how much is removed before reaching ground water, and ultimately waterways. Variations range from 10 percent to 90 percent removal, depending on soil types and other factors that change from site to site.

It is not wholly coincidence that the highest estimates of the septic tank threat often come from the agricultural sector, which is taking a pounding for its own large contributions of nitrogen to ground water

On the other hand, when you watch home after home going in on septic systems right next to streams and rivers going straight to the bay, you have to wonder about the reality of our goals to reduce nitrogen to save the estuary.

It is obvious we need to learn more about what septic tanks are really doing to ground water. Based on what we do know, it seems likely they are a significant problem, if not as large a one as discharges from sewage plants, runoff from farms and fallout from air pollution.

However, we have formal, if imperfect, programs to reduce those latter three sources. But septic systems are actually growing in impact. And water quality is only part of the reason we'd better get cracking on solutions to septic tanks.

For example, there is a baywide commitment to reduce nitrogen from sewage discharges to a certain level and cap it there, never allowing it to rise even as population grows.

If we stick to that, and meanwhile have no such cap on nitrogen from septic systems, they could become a huge and attractive loophole for developers. That would exacerbate more than a nitrogen problem, because septic systems lead to the sprawl development that already is gobbling up far too much farm and forest land across the bay region.

The whole idea of economic and environmentally sound growth is to steer population to areas where relatively high densities of people, and the roads and sewers and services to support them, already exist.

Septic systems drive development in the opposite direction. To satisfy health department standards for adequate filtration, the systems need large lots on prime, well-drained soils, the same places best suited for growing crops and forests.

To retrofit existing septic systems seems hugely impractical. The costs would probably run several thousand dollars per house, but alternatives do exist for new development.

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