BackgroundThe Colorado Department of Agriculture is charged withprotecting Colorado groundwater from contaminationfrom pesticides. In order to guide prevention activities,a study was conducted to assess the relative sensitivity ofthe state’s principal groundwater resources to pesticidecontamination.

Aquifer Sensitivity is defined as the relative ease withwhich a pesticide applied on or near a land surface canmigrate to the groundwater. Sensitivity is largely afunction of the physical characteristics of the overlyingarea. Sensitivity is not dependent on managementpractices or pesticide characteristics. Aquifer“vulnerability” considers both thesensitivity of the aquifer, as well as theland use, management, and pesticideproperties. This analysis of aquifersensitivity must be used with othersupporting information to determinewhere additional precautions must betaken to protect groundwater frompesticide contamination.

Aquifer Sensitivity FactorsA number of factors have beenidentified which may affect thesusceptibility of groundwater tocontamination from pesticides. Of themany possible factors, the followingfour primary factors were identified as

critical in describing the sensitivity of groundwater topesticide contamination in Colorado:

1) conductivity of exposed aquifers

2) depth to water table

3) permeability of materials overlying aquifers

4) availability of recharge for transport ofcontaminants

These selected factors incorporate the best data currentlyavailable for the entire state and incorporate importantaspects of Colorado’s unique climate and geology.

Relative Sensitivity of Colorado Groundwater to Pesticide Impact

Figure 1. Principal aquifers of Colorado

Fact Sheet #16October 1998

Factor 1) - Extent of Primary Exposed AquifersIn agricultural regions of Colorado, a number of aquiferssupply water for domestic, irrigation, and commercialuses. Between these primary aquifers are regions wheregroundwater supplies are inconsistent and/or provide lowwater yields. Conductivity of these priority aquifers ishighly variable, but overall is much higher than areas notunderlain by one or more of these principal aquifers.Therefore the presence or absence of one or more ofthese principal aquifers was selected as the indicator ofhigh conductivity aquifer areas.

Factor 2) - Depth to Water TableDepth to the water table affects the length of timerequired for a pesticide to reach the groundwater. Sincereasonably extensive data on depth to water table isavailable, depth to water table is incorporated directlyinto the sensitivity analysis.

Factor 3) - Permeability of Materials Overlying AquifersThe permeability of the materials overlying the aquiferaffects the time required for water to reach thegroundwater, an important consideration when dealingwith chemicals such as pesticides that break down overtime. Soil characteristics related to permeability includesoil texture, particle size distribution, soil structure, andhydrologic group. The hydrologic group designationdescribes runoff potential of a soil. Soils with high runoffpotential will accordingly have low infiltration potential.Because the hydrologic group designation includesconsideration of several factors important in controllingthe infiltration rate of a soil, it is felt that it carries moreinformation for an analysis at this scale than other single

soil parameters. Therefore, the hydrologic groupdesignation was chosen as the best availablerepresentation of the permeability of materialsoverlying the aquifers.

Factor 4) - Recharge AvailabilityThe amount of water available for transport of pesticide tothe groundwater is an important consideration inColorado’s semi-arid climate. Average annualprecipitation in Colorado’s agricultural areas ranges fromapproximately 7 to 17 inches. Low precipitation, coupledwith high evapotranspiration rates, leaves little moistureavailable for infiltration and subsequent aquifer recharge.Estimates of natural recharge rates in agricultural areas ofColorado are around 10 percent of precipitation orapproximately 1 inch/year. Estimates of recharge ratesfrom irrigated agriculture range from 5 to 30 inches/yeardepending on irrigation type, soil properties, andmanagement. Due to the relative abundance of rechargeunder irrigated agriculture compared with the limitednatural recharge supply in Colorado’s climate, thepresence or absence of irrigated agriculture waschosen as an indicator of recharge availability.

Map DescriptionA geographic information system (GIS) analysis wasconducted to incorporate the data layers into a sensitivitymap. A GIS is a computer system that allows comparisonand analysis of spatial data layers or “digital maps.” Eachlayer and its respective range of values are shown in Table1 and were used to develop the sensitivity map.

Table 1. Sensitivity data layers and corresponding factors of consideration.

Data Layer (Index) Value Range Factor of Consideration

Location of aquifer (AQU) 0-1 Conductivity of exposed aquifers

Water table depth (WTD) 1-4 Depth to water table

Soil permeability (SOIL) 1-4 Permeability of overlying materials

Available recharge (RCH) 0-1 Availability of recharge for transport

Water Table Depth IndexInformation on depth to watertable was obtained frompublished reportssummarizing water tableelevation surveys or fromreports of well measurementsand well logs. Water tabledepths were divided into 3categories for use insensitivity index calculation(Table 2). The map of thewater table depth index isshown in Figure 3.

Aquifer IndexThe aquifer map wasdeveloped from digitizedpublished reports of aquiferextent or from digitized generalgeologic maps (figure 2). Allareas overlying exposedprincipal aquifers wereassigned a value of 1. Otherareas are assigned a value of 0.

Figure 2. Extent of principal water table aquifers in Colorado(AQU Index)

Figure 3. Depth to water table index - (WTD Index)

100 km(62 miles)

Albers equal area projection. 1998

100 km(62 miles)

Albers equal area projection. 1998

1 (>50 ft) 3 (0-20 ft)2 (20-50 ft)

Soil indexSoil map units were reclassified based on their hydrologicgroup classification. In cases where a single map unitincludes soils with different hydrologic groupdesignations, the hydrologic group representative of themajority of land area was selected as most representativeof the map unit. The hydrologic groups were assigned

numerical indices based on their likelihood to transmitwater to underlying groundwater (Table 3). The map ofthe soil index is shown in figure 4.

Table 3. Relationship of SOIL index to hydrologic groups.

SOIL Hydrologic InfiltrationIndex Group Rates

1 D Very slow

2 C Slow

3 B Moderate

4 A High

Figure 4. Index of soil infiltration capacity in Colorado - (SOIL Index)

Table 2. Water table depth index interpretation. (WTD Index)

WTD Depth to InterpretationIndex Water Table

1 Greater than 50 feet Low sensitivity

2 20-50 feet

3 0-20 feet Higher sensitivity

100 km(62 miles)

Albers equal area projection. 1998

Very Slow Infiltration Capacity

Slow Infiltration Capacity

High Infiltration Capacity

Moderate Infiltration Capacity

Recharge IndexFor the western part of the state within the drainage basinof the Colorado River, location of irrigated lands wasobtained from a detailed analysis by the U.S. Bureau ofReclamation. For the remainder of the state, data onlocation of irrigated lands were obtained from satelliteimagery. For calculation of the aquifer sensitivity index,irrigated lands were assigned the value of 1, and non-irrigated lands were assigned to 0. (Figure 5)

Calculation of Aquifer Sensitivity Index (SENSITIVITY)A sensitivity range was then calculated and scaled to 1 to4 to obtain the index of SENSITIVITY (Table 4). Themap of the SENSITIVITY index is shown in figure 6.

Figure 5. Extent of irrigated areas in Colorado - (RCH Index)

Table 4. Sensitivity index and interpretation.

SENSITIVITY InterpretationIndex

1 Area of low recharge and/or aquifer conductivity

2 Low sensitivity

3 Moderate sensitivity

4 High sensitivity

100 km(62 miles)

Albers equal area projection. 1998

Interpretation of the Sensitivity MapThis map was developed with the objective of a regional-scale assessment of groundwater sensitivity to pesticidecontamination. The information presented in the mapsshould be used to support conclusions concerning areason a minimum scale of tens of miles. Areas that aredenoted on the map as having a low sensitivity maycontain individual fields overlying small areas of verysensitive groundwater. Additionally, groundwaterdelineated as having a low sensitivity may be susceptibleto contamination if irrigation or pesticide managementpractices promote leaching.

Sensitivity index values of 1 (green) represent areas thatare not irrigated and/or do not overlie highly conductiveaquifers. Conversely, sensitivity index values of 4 (red)represent areas where a very shallow water table in ahighly conductive aquifer coincide with at leastmoderately permeable soils that receive irrigation.Additionally, areas with shallow water tables but whichare not currently irrigated are also shown on the finalanalysis map since these areas might merit particularattention if brought under irrigation in the future.

The map is intended as a general guide in identifyingareas of the state in which groundwater, due to itshydrologic and geologic setting, is more or lesssusceptible to contamination from pesticide use. The analysis considers only the hydrogeologic setting.No consideration of actual pesticide use, crop patterns,management practices, etc. was attempted. Therefore,this analysis should be combined with knowledge ofother factors which might contribute to the overallvulnerability of the groundwater resource indevelopment of protection strategies and managementplans.

This report was prepared by Dr. Maurice Hall, Radford University, in cooperation with the Colorado Department ofPublic Health and Environment, Colorado State University Cooperative Extension, and the Colorado Department of Agriculture.

For more information or copies of the full report on this sensitivity analysis, contact Reagan Waskom of CSU at970/491-6103, Brad Austin of the Colorado Department of Public Health and Environment at 303/692-3572, or Robert P. Wawrzynski of the Colorado Department of Agriculture at 303/239-4151.