Water courses and ground water are contaminated chemically primarily due to industrial and agricultural activities. Industrial effluents and toxic chemical residues that are buried and which seep into water tables are the chief sources of contamination. Agricultural activities, mainly involving pesticides, herbicides and chemical fertilizers, are the cause of contaminated water run-offs brought about by rain or irrigation. Rivers and estuaries from which drinking water is drawn and purified have also to contend with contamination from boat and ship emissions of oil and petrol. Open areas of water may also, on occasion, be contaminated by precipitated particles in rainfall.