GIS based model for assessing groundwater
European User Conference
GIS based model for assessing groundwater pollution potential by pesticides
YE ZHAO, MARINA DE MAIOPOLITECNICO DI TORINO
Introduce
� Italy has a very high consumption of water, about 380 liters of water a day. Meanwhile, more than 85% of the drinking water in Italy is extracted from aquifer (Onorati et. al, 2006).
� The study area Vercelli field, which is situated on the river Sesia in the plain of the river Po, is an important centre for the cultivation of rice and maize.
Approximately 65% of the Approximately 65% of the study area is occupied by agricultural land, 27% by fruit crops, forest and lawn, and 8% by others (such as urban areas and water bodies). In this case, the most frequently detected groups of toxic organic chemicals is pesticides.
Study area
Pesticide Minimun
(µg/l)
Maximun
(µg/l)
Median
(µg/l)
Number of
detects
alachlor ND 1.2 0.0266 6
atrazine ND 2.2 0.0911 43
bensulfuron-methyl ND 0.23 0.005 3
bentazone ND 8.38 0.4172 37
Pesticides detected in groundwater
Agricultural practices
bentazone ND 8.38 0.4172 37
dimethenamid ND 2.26 0.1127 17
diazinon ND 0.49 0.0057 1
metolachlor ND 0.19 0.009 14
molinate ND 0.4 0.0143 5
quinclorac ND 4 0.0923 9
simazine ND 0.53 0.0526 45
terbumeton ND 0.09 0.001 1
terbuthylazine ND 14.3 0.2194 47
Introduce
Various attempts to evaluate groundwater vulnerability to surface contaminants have been made over the past two decades. Generally (Thapinta and Hudak, 2003), they can be classified as
1111、、、、 Direct observations of pesticides or other agricultural contaminants in groundwater Not cost-effective methods compared to other methods
3、、、、 Index methodshave been generated using a variety of ranking or scoring methods to produce qualitative or semi qualitative output. Thanks for the developing of geographic information systems (GIS), which is ideally suited to mapping and analyzing groundwater vulnerability factors over regions.
2、、、、 simulation methodsmodels help to understand the mechanism of pesticide leaching in soils towards groundwater, which are useful tools for assessing the risk of groundwater contamination resulting from the agricultural use of pesticides, in a relative local area.
Preparation of the input maps1
Sensitivity analysis2
Aquifer risk assessment3
Individual pesticide studies4
Preparation of the input maps
LanduseLanduseLanduseLanduseLanduseLanduseLanduseLanduse
slopeslopeslopeslopeslopeslopeslopeslope
infiltrationinfiltrationinfiltrationinfiltrationinfiltrationinfiltrationinfiltrationinfiltration
Water table Water table Water table Water table depthdepthdepthdepth
Water table Water table Water table Water table depthdepthdepthdepth
Preparation of the parameter maps
Rating Land use
5 Cereals, corn field
4 orchard, forest
3 Pasture, lake
2 urbanized areas
1 uncultivated
Land use and land cover was classified due to different usage patterns
Agricultural land covers much of the flood plain in the study area. These areas are the main sources of pesticides. The urban area is distributed among the farm field which contributed less than 8% of all the study area as shown.
Preparation of the parameter maps
The slope map was transformed from the elevation map with special analysis tool in GIS, rating from 1 to 5.
Topography is mainly flood plain in
Rating Slpoe (%)
5 0-2
4 2-5
3 5-10
2 10-15
1 >15
Topography is mainly flood plain in almost all the study area, more than 85% of the plain has the percent slope less than 2%.
Most of the area has a rating of 5 as the lower percent slope make water retain for a longer time, which allows a greater infiltration of recharge of water.
Preparation of the parameter maps
Rating Infiltration
1 0-50
2 50-80
3 80-130
4 130-160
5 >160
Rainfall map was obtained by interpolating a 10 years mean of annual precipitation (mm/year) from 14 representative rainfall stations in
Infiltration
14 representative rainfall stations in and around the study area. The infiltration map was then classified into ranges and assigned ratings from 1 to 5.
Preparation of the parameter maps
Rating Depth
5 0-4
4 4-8
3 8-12
2 12-16
1 >16
The location of the 25 wells was digitized to attribute the map of
depth to groundwater table with Kriging method of interpolation.with Kriging method of interpolation.The higher of depth the more time for the attenuation of pesticides, the pesticide usually has a great gap of half life between in soil and in water.
Sensitivity analysis and aquifer risk assessment
Sensitivity analysis are used to determine how important of every input variable to contribute the final risk of groundwater, with comparing the correlation coefficient between assigned ratings of input parameters and observed data from wells.
Landuse Depth of water table Infiltration Slope
Equationa=0.2867x+2.046 r2=0.6221
b=0.6208x+0.9028 r2=0.8672
c=0.4097x+1.5503 r2=0.4115
d=0.64x+0.6889 r2=0.6426
a: rating of landuse; b: rating of depth of water table; c: rating of infiltration; d: rating of slope; x: risk rating of observed wells of shallow aquifer
Sensitivity analysis and aquifer risk assessment
y1=0.5443x+1.4497 ; r2=0.9941
individual pesticide studiesIn fact, pesticides leaching into the groundwater was influenced by many factors such as molecular connectivity parameters Koc, degradation (soil half-life), solubility and molecular, the most important two are Koc and Dt50 (Fava et al., 2007; Fenolla et al., 2011). Koc and Dt 50 were used to calculate the leaching potential of each compound, expressed as Groundwater Ubiquity Score (GUS)indices as follow.
GUS >2.8 : potential leaches (L)1.8< GUS <2.8 : transient properties (T) GUS <1.8 : non-leaches (NL)
Pesticide
Number of
detects CUS index
alachlor 6 2.19
atrazine 43 3.75
bensulfuron-methyl 3 2.07
bentazone 37 2.55
dimethenamid 17 2.19
diazinon 1 1.14
metalaxyl 0 2.11
metolachlor 14 3.32
molinate 5 2.49
simazine 45 3.35
terbuthylazine 47 3.07
0
10
20
30
40
50
60
0 1 2 3 4
nu
mb
er o
f d
etec
ted
GUS index
individual pesticide studiesatrazinesimazine
terbuthylazine bentazone
Conclusions
•Four parameters were considered
• aquifer risk was assessed
Land use, depth of water table infiltration and slope
water table depth was most significant factor among four
• aquifer risk was assessed
linear method can be considered as the most stable methodology, as it do not amplify the error of single parameter
•Individual pesticide was studiedGUS is a important index to indicate the leaching potential of pesticide
with the time pass, the pesticide can be redistributed and degraded slowly