Table 3. Summary of pesticides detected in surface water and Groundwater in the Potomac River Basin

[MDL: Method Detection Limit; MRL: Minimum Reporting Limit; (E): concentration estimated; ug/L: micrograms per liter; <:less than]


  Surface Water Groundwater
Chemical Name MDL or MRL (µg/L) Number of Samples Collected Number of Sites Where Pesticide Percent of Samples Having Detectable Concentration 90th Percentile Concentration (µg/L) Maximum Reported Concentration (µg/L) Number of Sites Sampled Number of Sites Where Pesticide Detected 90th Percentile Concentration (µg/L) Maximum Reported Concentration (µg/L)
Samples analyzed by gas chromatography with detection by mass spectrometry (Zaugg and others, 1995) (112 surface-water sites sampled for this analysis method)
Alachlor 0.002 279 14 24.4a 0.029 3.100 105 1a <0.002 0.005
Atrazine .001 279 92 88.2a .730 25.0 (E) 105 36 .250 1.20
Azinphos-methyl .001 275 4 3.6a <.001 .130 (E) 104 0 < .001 < .001
Benfluralin .002 277 1 0.7 <.002 .030 104 0 < .002 < .002
Butylate .002 277 2 0.7 <.002 .018 104 0 < .002 < .002
Carbaryl .003 279 23 28.3a .064 2.00 (E) 105 0 < .003 < .003
Carbofuran .003 279 7 5.7a <.003 .460 (E) 105 0 < .003 < .003
Chlorpyrifos .004 277b 12 17.0a .008 .041 104 0 < .004 < .004
Cyanazine .004 279b 17 23.3a .078 3.00 105 0 < .004 < .004
DCPA .002 277c 9 12.6a .002 .045 104c 1a < .002 .001 (E)
DCPA .002 277c 9 12.6a .002 .045 104c 1a < .002 .001 (E)
p,p'-DDE .006 277c 6 2.2a <.006 .023 104c 10a < .006 .001 (E)
Desethylatrazine .002 279b 80 79.6a .150 .690 (E) 105b 38a .140 1.40 (E)
Diazinon .002 277 21 30.0a .071 1.40 104 0 <.002 <.002
Dieldrin .001 277 2 0.7 <.001 .018 104 0 <.001 <.001
2,6-Diethylanaline .003 277b 1 0.4a <.003 .014 (E) 104 0 <.003 <.003
EPTC .002 277 5 3.6a <.002 .012 104 3a <.002 .013
Ethoprop .003 279 1 0.4 <.003 .027 105 0 <.003 <.003
Fonofos .003 277 3 2.2a <.003 .084 104 0 <.003 <.003
gamma-HCH .004 277 2 1.1 <.004 .025 104 0 <.004 <.004
Linuron .002 277 5 10.5 .009 1.40 104 0 <.002 <.002
Malathion .005 277 5 5.1a <.005 .410 104 0 <.005 <.005
Methylparathion .006 277 4 3.6a <.006 .080 104 0 <.006 <.006
Metolachlor .002 279b 83 84.9a .990 23.0 (E) 105 20a .009 .460
Metribuzin .004 277 9 11.6a .008 .160 104 2a <.004 .012
Napropamide .003 279 2 0.7 <.003 .024 105 0 <.003 <.003
Pebulate .004 277 0 0.0 <.004 <.004 104 1a <.004 .004 (E)
Pendimethalin .004 277 5 13.4a .022 .320 104 0 <.004 <.004
Prometon .018 279c 68 71.7a .077 1.70 (E) 105 c 16 a .018 .900 (E)
Propachlor .007 279b 1 1.4a <.007 .046 105 0 <.007 <.007
Simazine .005 279b 83 84.9a .510 4.40 105 23a .065 .210
Tebuthiuron .010 279c 22 21.5a .010 .220 105 2a <.010 .086
Terbucil .007 274b 7 6.9a <.007 .130 (E) 104c 1a <.007 .260 (E)
Trifluralin .002 277 9 4.7 <.002 .027 104 0 <.002 <.002
Samples analyzed by high-performance liquid chromatography with detection by ultraviolet spectroscopy (Werner and others, 1996) (8 surface-water sites sampled for this analysis method)
Chlorothalonil .035 126 1 0.8a <.035 .100 82 0 <.035 <.035
2,4-D .035 127 4 19.7a .340 2.80 82 0 <.035 <.035
Dicamba .035 127 0 0.0 <.035 <.100 82 1 <.035 .070
Dichlobenil .020 128 0 0.0 <.020 <.100 82 1a <.020 .100 (E)
Dichlorprop .032 127 1 0.8 <.032 .150 82 0 <.032 <.032
Diuron .020 128b 2 14.8a .090 .690 (E) 82 0 <.020 <.040
MCPA .050 127 1 1.6 <.050 1.30 82 0 <.050 <.050
Oryzalin .019 128 1 6.3a <.019 1.90 82 0 <.090 <.038
Propoxur .035 118 0 0.0 <.035 <.035 77 1 <.035 .060
Triclopyr .050 127 2 3.9a <.050 .830 82 0 <.050 <.050

aSome detections of estimated values were reported for this pesticide.
bLess than 50 percent of detectable concentrations were estimated below the MDL or MRL.
c50 percent or more of detectable concentrations were estimated below the MDL or MRL.