▁bio activity . ▁The ▁term ▁cont amin ant ▁is ▁often ▁used ▁inter change ably ▁with ▁poll ut ant , ▁which ▁is ▁a ▁subst ance ▁that ▁has ▁a ▁det r iment al ▁impact ▁on ▁the ▁surrounding ▁environment . ▁ ▁Wh ilst ▁a ▁cont amin ant ▁is ▁sometimes ▁defined ▁as ▁a ▁subst ance ▁present ▁in ▁the ▁environment ▁as ▁a ▁result ▁of ▁human ▁activity , ▁but ▁without ▁harm ful ▁effects , ▁it ▁is ▁sometimes ▁the ▁case ▁that ▁to xic ▁or ▁harm ful ▁effects ▁from ▁cont am ination ▁only ▁become ▁apparent ▁at ▁a ▁later ▁date . ▁ ▁The ▁" medium " ▁( e . g . ▁soil ) ▁or ▁organ ism ▁( e . g . ▁fish ) ▁affected ▁by ▁the ▁poll ut ant ▁or ▁cont amin ant ▁is ▁called ▁a ▁re ceptor , ▁whilst ▁a ▁sink ▁is ▁a ▁chemical ▁medium ▁or ▁species ▁that ▁retain s ▁and ▁interact s ▁with ▁the ▁poll ut ant ▁e . g . ▁as ▁carbon ▁sink ▁and ▁its ▁effects ▁by ▁micro bes . ▁ ▁Environment al ▁indic ators ▁ ▁Chem ical ▁measures ▁of ▁water ▁quality ▁include ▁dissol ved ▁o xygen ▁( DO ), ▁chemical ▁o xygen ▁demand ▁( CO D ), ▁bio chem ical ▁o xygen ▁demand ▁( BO D ), ▁total ▁dissol ved ▁sol ids ▁( T DS ), ▁p H , ▁nut ri ents ▁( nit r ates ▁and ▁ph osph orus ), ▁heavy ▁met als , ▁soil ▁chemical s ▁( including ▁co pper , ▁z inc , ▁cad m ium , ▁lead ▁and ▁mer cur y ), ▁and ▁p