Nitrates,
nitrites, and other nitrogen compounds can contaminate drinking water. They
have been associated with a higher risk of cancer and other diseases. Nitrogen
fertilizers, including manure, are the culprits that lead to this pollution in
both surface water and groundwater. The goal of managing agricultural runoff is
to reduce the amount of nitrogen, other nutrients like phosphorus, and sediment
in drinking water and in water bodies. Nitrate pollution is generally common
wherever fertilized agriculture is common.
Saturated Buffers
Saturated buffers
are the easiest and cheapest way to address the threat of nitrate pollution in
drinking water, but many question whether they are enough. USDA defines a
saturated buffer:
“A saturated buffer is a vegetated, riparian buffer in
which the water table is artificially raised by diverting much of the water
from a subsurface drainage system along the buffer to reduce nitrate loading to
surface water through enhanced denitrification.”
The USDA’s Agricultural Research Service explains the
composition of saturated buffers as follows:
“(1) a non-perforated drain pipe, (2) water control
structure, (3) distribution pipe, and (4) a vegetated buffer. The tile drained
water is directed to the control structure via the drain pipe. That water is
then diverted into the perforated distribution pipe, where it is slowly pushed
through the vegetated buffer. While crossing the buffer, denitrification occurs
(a microbial facilitated process of nitrate being converted to nitrogen gas)
along with nitrate uptake by the plants within the buffer.”
They note that on
average, saturated buffers remove 42% of the nitrate in the runoff water. That
amount can increase with longer piping networks. Cost is dependent on size.
Saturated buffers generally do not require maintenance. There are, however,
some design considerations that the USDA NRS explains below:
“First, a buffer and tile outlet should already be in
place. The vegetated buffer must be at least 30 feet wide and the tile outlet
must drain enough area to provide sufficient flow. Second, soils must promote
gradual, lateral movement to the water body (e.g. loam soil). Clayey soils may
not allow movement, and sandy soils to gravel will result in too quick of
movement. There also needs to be at least 1.2% of organic matter within the top
2.5 feet of soils to encourage denitrification. Carbon enhancement, using
woodchips, has been suggested if organic matter lacks. Third, the buffer should
be lower than the adjacent field to avoid inundation. If the slope is not
ideal, seasonal, manual adjustment of the water control structure can allow
adequate drainage. Lastly, stream banks should be stable and not exceed 8 feet
in height. This provision ensures no seepage through the stream bank which can
results in bank failures.”
The piping diverts the water to run laterally along the buffer
zone where it leaks off slowly via the perforated pipes. The piping is run parallel
to drainage ditches. This saturates the soil and allows vegetation and
denitrifying soil organisms to remove nitrates from the water before it is discharged
into nearby streams. Onsite soil evaluation is important for saturated buffers.
As noted, there should be sufficient organic matter to power the denitrification
process and a restrictive layer to keep the water from running too fast into deeper
layers before it is denitrified. The Natural Resources Conservation Service
(NRCS) Conservation Practice Standard (CPS) Saturated Buffer (Code 604) guides
saturated buffer construction. The buffers cost about $8000 per system. The distribution
system is somewhat similar to a distribution system for a septic system leach
field. The soil suitability is also similar in terms of ideal soil infiltration
loads.
Some say
saturated buffers do not do enough to improve water quality and that more control
is needed to further reduce nitrate contamination. Since most saturated buffers
did not improve water quality enough to meet standards, the Iowa state government
and the NRCS lowered those standards, which allowed saturated buffers to be
built where they are less effective, say detractors. Some prefer regulation
over building more and more partial solutions like saturated buffers and they also
complain that in Iowa, the Ag Industry is too powerful to accept any more
stringent regulations.
Saturated Buffers: A nutrient removal option for farmers
Bioreactors
According to
Vanya North of Practical Farmers of Iowa:
“A bioreactor is a buried trench on the edge of a field
that is filled with woodchips. Agricultural drainage tiles direct water runoff
from fields into the woodchips where bacteria convert the nitrates present in
the water into nitrogen gas, the most abundant naturally occurring gas in
earth’s atmosphere.”
“Like saturated buffers, bioreactors are an edge of field
practice, meaning they do not impact in field management. A two-stage
bioreactor can also filter out high levels of phosphorous from field runoff.”
There are incentives available to help farmers pay for all
or part of saturated buffers and bioreactors:
“According to the Ohio State University Extension, costs
of either are site specific and depend on the size of the control structure and
the length of the saturated buffer, but a minimum of 30 feet across is the set
requirement for optimal drainage. The average cost for materials and
installation was about $3,720 in 2021 and $1000-1500 more for a bioreactor
depending on if you need to contract equipment such as excavators. Once
installed, a saturated buffer is relatively self-sustaining while a bioreactor
will need its woodchips changed out every 10-15 years depending on the rate of
decomposition.”
Bioreactors are often used in conjunction with saturated
buffers to enable very good water quality improvements. The table below
compares the benefits of saturated buffers and bioreactors.
Bioreactors
enhance the denitrification process with the addition of woodchips to feed the
denitrifying bacteria. Experiments using bioreactors began around 2006. They
not only reduce the amount of runoff to local waterways, but if implemented on
a wide enough scale they can, along with saturated buffers, reduce the amount
of nutrients getting into the Mississippi River and the Gulf of Mexico.
According to a 2016 USDA blog:
“We’re really excited about the potential to spread this
technology across the Mississippi River Basin,” said Dr. Wayne Honeycutt,
deputy chief for science and technology at NRCS. “When paired with nutrient
management, cover crops and no-till practices, denitrifying bioreactors are a
fantastic line of defense for subsurface nitrates.”
The average bioreactor can remove 35-50% of nitrates from
the water flowing through it, without restricting drainage. That is pretty
similar to saturated buffers but at a slightly higher cost. One thing the NRCS
learned is that bioreactors should not be built too large to control the water
flowing through them. The average bioreactor model costs $8,000-$12,000,
according to NRCS.
Nabbing Nitrates Before Water Leaves the Farm: Bioreactors
References:
Iowa
is trying to deal with farm runoff using 'saturated buffers.' Is it enough?
Emily Haavik. NPR. All
Things Considered. December 17, 2024. Iowa
is trying to deal with farm runoff using 'saturated buffers.' Is it enough? :
NPR
What
are Saturated Buffers? USDA. Agricultural Research Service. What
are Saturated Buffers? : USDA ARS
Conservation
Practice Overview. Saturated Buffer
(Code 604). USDA. September 2020. Practice
Overview for CPS Saturated Buffer (Code 604)
Saturated
Buffers and Bioreactors: What Are They and Which Is Right For You? Vanya North.
Practical Farmers of Iowa. November 3, 2022. Saturated
Buffers and Bioreactors: What Are They and Which Is Right For You? - Practical
Farmers of Iowa
Bioreactors
Form a Last Line of Defense against Nitrate Runoff. USDA. February 26, 2016. Bioreactors
Form a Last Line of Defense against Nitrate Runoff | Home
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