The gullies and unprotected stream banks captured by EWG’s aerial survey are the beginnings of a long train of polluted water and degraded soils that stretches from Minnesota to the dead zone in the Gulf of Mexico.

Polluted Runoff

The sheer amount of runoff can be enormous. A half inch of water running off a 40-acre crop field — small by Iowa standards — amounts to 543,000 gallons, almost enough to fill an Olympic-sized pool (660,000 gallons). The May 25 storm produced runoff ranging up to 10,500 gallons per acre in the area EWG surveyed. The more intense May 12 storm produced up to 19,800 gallons per acre, and downpours often result in far larger volumes.

Gullies like those observed in Marshall County are pipelines that also carry mud, fertilizers, pesticides, manure — essentially anything that is applied to a crop field — into streams.

The pipeline is especially direct when a gully runs right into a stream or ditch, as was often the case on the fields EWG surveyed. The NRCS scientists estimate that 50-to-90 percent of the soil in such gullies ends up in a stream.10 The amounts can be huge. A gully 3 inches deep, 2 feet wide and 100 yards long represents 6.8 tons of eroded soil, more than the load of a typical single-axle dump truck.

Farmers routinely fill in gullies in order to smooth out their fields. This keeps supplying more and more soil to the gully — soil that ends up in a stream when the next storm hits. Cumulative soil losses increase dramatically as gullies are refilled and eroded over and over again.11

This sediment — mud — is itself a pollutant, as well as a carrier of other contaminants. Muddy water degrades fish habitat and clogs water treatment plants. Sediment is the most widespread pollutant damaging rivers and streams, according to the U.S. Environmental Protection Agency (EPA), and agriculture is the primary source.12

The water and mud running off of crop fields carry with them a potent stew of pesticides, fertilizers, manure, bacteria and other pollutants.

According to the Iowa Policy Project, Iowa farmers apply 1.7 billion pounds of nitrogen and 635 million pounds of phosphorus to corn and soybean fields each year.13 USDA’s National Agricultural Statistics Service (NASS) reported that Iowa farmers in 200514 — the latest year for which data are available — used 14 different herbicides, including 2,4-D, acetochlor, atrazine, dicamba and glyphosate. Iowa also produces about 286 million tons of manure each year, most of which ends up on crop fields.15

In a single seven-month period in 2008, an interdisciplinary team of scientists at Iowa State University studying runoff from small watersheds measured cumulative sediment loads of 10.8 tons per acre and phosphorus losses of 30 pounds per acre.16

This brew of fertilizers, manure, pesticides, bacteria and mud does serious harm to streams, lakes and rivers. The organic matter in rich topsoil, along with the nitrogen and phosphorus in fertilizers, spawns noxious algal blooms downstream and damages fisheries. The algal blooms kill fish by reducing the amount of oxygen in the water. Outbreaks of blue-green algae (cyanobacteria), which release chemicals that are toxic to people and animals, are particularly harmful.17&18 Fields treated with manure commonly shed large amounts of E. coli, a type of bacteria used to indicate fecal contamination. EPA guidelines consider water to be unsafe for swimming if there are more than 126 “colony forming units” or “cells” of E. coli in 100 milliliters (ml) of water.19 Agricultural runoff often contains much higher numbers of E. coli. One study measured 86,645 cells per 100 ml in water running off a crop field in Iowa.20