This is not a full list of tap water impurities but they certainly are the most common.
1
1,1,1,2-Tetrachloroethane
1,1,1-Trichloroethane
1,1,2,2-Tetrachloroethane
1,1,2-Trichloroethane
1,1-Dichloroethane
1,1-Dichloroethylene
1,1-Dichloropropanone
1,1-Dichloropropene
1,2 Dibromo-3-chloropropane (DBCP)
1,2,3-Trichlorobenzene
1,2,3-Trichloropropane
1,2,4-Trichlorobenzene
1,2,4-Trimethylbenzene
1,2-Dibromoethylene
1,2-Dichloroethane
1,2-Dichloropropane
1,3,5-Trimethylbenzene
1,3-Dichloropropane
1,3-Dichloropropene
1,4-Dioxane
2
2,2-Dichloropropane
2,4,5-T
2,4,5-TP (Silvex)
2,4-D
2,4-DB
2,4-Dinitrotoluene
2-Hexanone
2-Nitropropane
25-Manganese-54
3
3-Hydroxycarbofuran
A
Acenaphthene
Acenaphthylene
Acetochlor
Acetone
Acrylonitrile
Alachlor (Lasso)
Aldicarb
Aldicarb sulfone
Aldicarb sulfoxide
Aldrin
Alpha Chlordane
Alpha particle activity
Alpha particle activity (excl radon and uranium)
Alpha particle activity (incl. radon & uranium)
Alpha particle activity (suspended)
alpha-Lindane
Aluminum
Ammonia
Aniline
Anthracene
Antimony (total)
Aroclor 1016
Aroclor 1221
Aroclor 1232
Aroclor 1242
Aroclor 1248
Aroclor 1254
Aroclor 1260
Arsenic (total)
Asbestos
Atrazine
B
Barium (total)
Baygon (Propoxur)
Bentazon (Basagran)
Benzene
Benzo[a]anthracene
Benzo[a]pyrene
Benzo[b]fluoranthene
Benzo[b]fluoranthene & Benzo[k]fluoranthene
Benzo[g,h,i]perylene
Benzo[k]fluoranthene
Beryllium (total)
beta-Lindane
Bromacil
Bromate
Bromide
Bromobenzene
Bromochloroacetic acid
Bromochloromethane
Bromodichloroacetic acid
Bromodichloromethane
Bromoform
Bromomethane
Butachlor
Butyl Acetate
Butyl Benzylphthalate
Butylate (Sutan)
C
Cadmium (total)
Carbaryl
Carbofuran
Carbon disulfide
Carbon tetrachloride
Chloramben
Chloramine
Chlorate
Chlordane
Chlorine dioxide
Chlorite
Chlorodibromoacetic acid
Chloroethane
Chloroform
Chloromethane
Chloropicrin
Chromium (hexavalent)
Chromium (total)
Chrysene
cis-1,2-Dichloroethylene
Combined Radium (-226 & -228)
Combined Uranium (mg/L)
Combined Uranium (pCi/L)
Copper
Cyanazine (Bladex)
Cyanide
D
Dacthal
Dalapon
Desethylatrazine
Desisopropylatrazine
Di(2-Ethylhexyl) adipate
Di(2-ethylhexyl) phthalate
Di-n-butylphthalate
Dibenz[a,h]anthracene
Dibromoacetic acid
Dibromochloromethane
Dibromomethane
Dicamba
Dichloroacetic acid
Dichloroacetonitrile
Dichlorodifluoromethane
Dichlorofluoromethane
Dichloroiodomethane
Dichloromethane (methylene chloride)
Dichlorprop
Dieldrin
Diethylphthalate
Dimethylphthalate
Dinoseb
Diquat
Diuron
E
Endosulfan I
Endothall
Endrin
EPTC (Eptam)
Ethion
Ethyl ether
Ethyl-t-butyl ether (ETBE)
Ethylbenzene
Ethylene dibromide (EDB)
F
Fluoranthene
Fluorene
Foaming agents (surfactants)
Formaldehyde
G
Glyoxal
Glyphosate
Gross beta particle & photon emitters (man-made)
Gross beta particle activity (dissolved)
Gross beta particle activity (mrem/yr)
Gross beta particle activity (pCi/L)
Gross beta particle activity (suspended)
H
Heptachlor
Heptachlor epoxide
Hexachlorobenzene (HCB)
Hexachlorobutadiene
Hexachlorocyclopentadiene
Hydrogen sulfide
I
Indeno[1,2,3-cd]pyrene
Iodomethane
Isophorone
Isopropyl alcohol
Isopropyl ether
Isopropylbenzene
L
Lead (total)
Lindane
Lithium
M
m- & p- Xylene
m-Dichlorobenzene
m-Xylene
Manganese
MBAS
Mercury (total inorganic)
Methiocarb
Methomyl
Methoxychlor
Methyl ethyl ketone
Methyl isobutyl ketone
Methyl methacrylate
Metolachlor
Metribuzin
Molinate (Ordram)
Molybdenum
Monobromoacetic acid
Monochloroacetic acid
Monochlorobenzene (Chlorobenzene)
MTBE
N
n-Butylbenzene
n-Hexane
n-Nitrosodiphenylamine
n-Propylbenzene
Naphthalene
Nitrate
Nitrate & nitrite
Nitrite
Nitrobenzene
O
o-Chlorotoluene
o-Dichlorobenzene
o-Xylene
Oil & Grease (total)
Orthophosphate
Oxamyl (Vydate)
P
p,p’-DDT
p-Chlorotoluene
p-Dichlorobenzene
p-Isopropyltoluene
p-Xylene
Paraquat
Pentachlorophenol
Perchlorate
Phenanthrene
Phenols
Phosphate
Phosphorus
Picloram
Potassium-40 (total)
Prometon (2,4-bis -6-(isopropylamino))
Propachlor
Pyrene
R
Radium-226
Radium-228
Radon
S
sec-Butylbenzene
Selenium (total)
Silver (total)
Simazine
Strontium
Strontium-90
Styrene
Sulfate
T
tert-Butylbenzene
Tetrachloroethylene
Tetrahydrofuran
Thallium (total)
Thiobencarb (Bolero)
Toluene
Total haloacetic acids
Total polychlorinated biphenyls (PCBs)
Total trihalomethanes (TTHMs)
Toxaphene
trans-1,2-Dichloroethylene
Tribromoacetic acid
Trichloroacetic acid
Trichloroethylene
Trichlorofluoromethane
Trichlorotrifluoroethane
Trifluralin
Tritium
U
Uranium-234
Uranium-235
Uranium-238
V
Vanadium
Vinyl chloride
Information on each of these impurities can be found here.