Research questions. â· What is the spatial distribution of groundwater travel times from residential septic tanks to nearby streams? â· What are the spatial ...
Three-dimensional modeling of subsurface nitrogen transport at the watershed scale a,b
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c
e
b,d
a,b
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Zhengtao Cui , Claire Welty , Arthur J. Gold , Peter M. Groffman , Andrew J. Miller , Elvis J. Andino-Nolasco , Michael L. Barnes , and Sujay S. Kaushal a b Department of Chemical, Biochemical and Environmental Engineering, UMBC, Baltimore, MD; Center for Urban Environmental Research and Education, c d UMBC, Baltimore, MD; Department of Natural Resources Science, University of Rhode Island, Kingston, RI; Department of Geography and Environmental e f Systems, UMBC, Baltimore, MD; Cary Institute of Ecosystem Studies, Millbrook, NY; Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD; Research questions
Stream groundwater N flux vs. simulation time
Groundwater flow path and residence times
I What is the spatial distribution of groundwater travel times from residential septic tanks to nearby streams? I What are the spatial patterns of nitrogen contamination and denitrification in groundwater resulting from septic tank effluent discharged from multiple sources? I What are the effects of effluent from multiple distributed septic tanks on stream-bed nitrogen concentrations?
Minimum: 3 years, Maximum: 100 years, Mean: 34 years 100
Red Run subwatershed "Sunnydale"
60
60 40 31 22 17 15 17 1619 14 11 10 6 7 3
41
40 36
37 29
29
24 2223 20 21 16 16
22 19 19 16 16
45 38
21
26
25
23 23 20
2021
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53
I Watershed area: 0.23 km . I No storm water management; the stream drains through underground pipes I 54 houses, most were built in the mid 1960s. I All houses are served by septic systems.
55 53
49
40
0
76
74
20
2
81
80
80
86
House numbers
920 629 675 700 694 632 470
427 481 412 532 540 212
439 355 159
581
667
813 563
560 87 124 127 122 191 144 271 112 303 167 186 212 202 154 245 333 297 247 218 256 309 402 109 125 301 433 186 398 296
167
37
118 26
House locations and hydrogeology
1000 900 800 700 600 500 400 300 200 100 0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53
Distance (m)
Minimum: 26 m, Maximum: 920 m, Mean: 340 m
House numbers
Stream bed N and conservative tracer concentrations at 160 years 3
6
1-4 November 2015
Conc. (mg/L as N)
4 3
NO− 3 NH+ 4 Conservative
160 years
Flow direction
2
2
160 years
Flow direction
1
1 0
Conservative tracer flux
Nitrogen flux
160 years
100
200 300 Simulation time (years)
400
Summary I We have illustrated the application of a 3D multispecies reactive transport code at the watershed-scale. I In deep groundwater we observed: . Nitrate concentrations attenuated to less than 10 mg N/L within 5 m from the source. . Conservative tracer prediction has greater errors than shallow groundwater near the stream due to longer travel times and distances in deep groudnwater. . Low denitrification rates due to low level of DOC and biomass. I Nitrate can behave as conservative tracer under some conditions. . Low nitrate concentrations . Short travel times. I Stream groundwater N flux reaches steady state at about 400 years. I At 50 - 60 years . Nitrate-N flux will continue to increase until steady state is reached. . Total stream nitrate-N flux increases at 3.4 g N/day/year.
3, (3 35) 1, (2 35) 9, (2 35) 7, (2 35) 6, (2 36) 4, (2 36) 3, (2 37) 2, (2 38) 1, (1 39) 9, (1 39) 8, (1 40) 8, (1 42) 8, (1 44) 7, (1 45) 6, (1 46) 6, (1 48) 6, (1 50) 5, (1 51) 5, 53 )
(3
7, (4 12) 6, (4 13) 6, (4 15) 4, (4 15) 2, (4 15) 1, (4 16) 1, (4 18) 0, (4 19) 0, (3 21) 9, (3 22) 9, (3 24) 8, (3 25) 8, (3 27) 7, (3 28) 7, (3 30) 6, (3 31) 5, (3 32) 5, (3 34) 4, 35 )
0
(4
I The underlying Piedmont physiographic province is divided into 4 geologic layers: (1) 1 m soil (cyan); (2) 14 m saprolite (yellow); (3) 3 m weathered bedrock (blue); (4) up to 42 m bedrock (red). I The 3D groundwater flow code ParFlow.CLM (Maxwell & Miller, 2005) and the multispecies reactive transport model (Cui et al., 2014) were applied to the domain. I Domain size: 480 m x 960 m x 60 m. I Grid resolution: 10 m x 10 m x 1 m; 276,480 total number of cells I Septic tanks are 2-3 m below land surface. + − I Nitrogen input is 33 g /day/home (1% NH4 , 99% NO3 ).
Conc. (mg/L as N)
5
NO− 3 + NH4 Conservative
Total flux (g/day)
Travel time (years)
100
100
98
120 110 100 90 80 70 60 50 40 30 20 10 0
Main stream chainage
Left branch chainage
Main stream
Left branch
GSA Annual Meeting 2015
Cui, Z., Welty, C., Maxwell, R.M., 2014. Modeling nitrogen transport and transformation in aquifers using a particle-tracking approach. Computers & Geosciences 70, 1-14. Maxwell, R.M., Miller, N.L., 2005. Development of a Coupled Land Surface and Groundwater Model. Journal of Hydrometeorology 6, 233-247.
Baltimore, MD
