Call Before You Dig!
Eric Regensburger
Montana DEQ
Method for EstimatingAttenuation of Nutrients
from Septic Systems(MEANSS)
(OR THE ARTIST FORMERLY KNOWN AS “SNoRES”)
Method for EstimatingAttenuation of Nutrients
from Septic Systems(MEANSS)
(OR THE ARTIST FORMERLY KNOWN AS “SNoRES”)
WHY MEANSS??
• Estimate septic loading for TMDL’s• Estimate septic loading for Nutrient
Trading (DEQ-13)• Need for a simple method that uses
site-specific info known to control denitrification
• Lack of available tools for estimating septic contributions to surface water……
OTHER TOOLS• ArcNLET: GIS based, ground water travel
time, user-defined denitrification• STUMOD: Spreadsheet, detailed
chemical/physical process, can use all default or site-specific parameter, N reduction below drainfield only
• SWAT: Complex model with biozone function (N reduction below drainfield only), can add user defined denit. rate
• WAG – Wild A#% Guess (my favorite)
PARAMETERS
• Three Variables:– Hydrologic Soil Group (HSG) at drainfield– Distance to nearest perennial surface water– HSG in 100 foot riparian buffer
• Variables receive %N reduction score• Add up the 3 values for total reduction• Assumes the complete volume of
effluent reaches surface water (SW)• Assumes simple flow direction to SW
DATA SOURCES
• Drainfield location from GIS: search “Montana Structures Framework” (drainfield location ~ residence)
• HSG is from NRCS STATSGO(2) OR SSURGO– HSG used to classify runoff potential– 4 HSGs – A,B,C,D: low to high clay content
(<10, 10-20, 20-40, >40 %).• Distance to receiving/perennial SW is
from GIS
A LITTLE MORE DETAIL…..• Denitrification occurs in soils with high
organic content, anoxic, and time for reaction to occur >>> HSG best commonly available data to estimate that.
• Travel distance easier to measure than travel time.
• Improve MEANSS accuracy:– Measure soil carbon content– Measure travel time instead of distance– Accurate flow direction for distance
Percent Nitrogen Load
Reduction
Soil Type @ Drainfield
Soil Type within 100’ of surface
water
Distance to surface water
(ft)
0 A A 0 – 100
10 B 101 – 500
20 C B 501 – 5,000
30 D C 5,001 – 20,000
50 D 20,001+
Stream
Drainfield
Group B Soil
Group C Soil
450 ft
100 ft
C soil @ d.f. = 20% B soil @ riparian =20%450 ft. to stream = 10%Reduction: 20+20+10=50%
COMPARISONS/VALIDATION
• Compared to 5 studies that measured N in ground water– Montana(2), Wisconsin(2), Nevada(1)– MEANSS estimated N load was 68%, 74%,
82%, 83% and 120% of the comparable loads measured in studies.
• In process of validating to a SWAT model for Prickly Pear drainage south of Montana City
MEANSS RESULTS
Watershed
# Septics Used in Analysis
% TN entering surface water
Lower Gallatin 15,871 56Bitterroot 15,934 33Little Blackfoot 326 47Flint Creek 1,623 36Helena 9,090 48Bison Creek 74 29Dayton Area 390 66
THE END ….