This text contains information derived from Tom Evans' paper, A Spatial and Statistical Assessment of the Vulnerability of Texas Groundwater to Nitrate Contamination. His approach and results were summarized as follows:

1. Nitrate concentration in groundwater is used as a surrogate for the groundwater vulnerability. He referred to a paper (Kolpin, 1993) which statistically relates occurrence of detecting nitrate and pesticide/herbicide in groundwater of Mid West. He defined probability of occurrence of nitrate concentration higher than defined threshold values (0.1, 1, 5 and 10 mg/L) as a measure of vulnerability.
2. The author thinks that the statistically defined vulnerability has advantages over index based vulnerability (e.g. DRASTIC method) as
   • it has a certain physical background in the parameter, and
   • it is quantitative and therefore allows further quantitative interpretation of the vulnerability
3. Measurements of nitrate in the wells throughout Texas since the end of last century (>60000 data) were used to determine probability of detecting nitrate concentration higher than defined concentrations. The data source is Texas water development board groundwater resource system. The author omitted data before 1963 and other less reliable data from his analysis.
4. The most important finding is that chance of detecting high nitrate concentration was lower in the alluvial part of Texas (East and South Texas) even though DRASTIC index in that area was higher than the rest of Texas. On the other hand, higher chances of nitrate detection is observed in west Texas where DRASTIC index was low.
5. The author tried to correlated his (nitrate based) vulnerability with several environmental factors. He considered following four factors:
   • Average annual precipitation
   • Soil thickness
   • Total organic carbon content across the soil layer
   • Estimated nitrate fertilizer use in the area
   Precipitation data were obtained from a commercial source. STATSGO was used for the soil property data. Fertilizer use was estimated from fertilizer sales, which was published by USEPA.
6. He found a statistically significant correlation between precipitation and his vulnerability, but not with other three factors.

The approach of the original paper can be expanded for this term project. Following projects can be relevant.

1. Use data other than nitrate to come up with a estimate of vulnerability similar to Tom's. Chemicals whose property is different from nitrate (inorganic, positively charged, small size) is preferred.
2. Tom found correlation between his vulnerability and precipitation. Precipitation may be strongly related to one factor in calculating DRASTIC index, which is recharge. He also examined effect soil properties but not vadose zone and aquifer properties. There might be some correlation between those and vulnerability.