Question

contamination of drinking water can happen covered in drain also

Answer 1

Yes your answer correct.

Answer 2

Contamination of drinking water sources by sewage can occur from raw sewage overflow, septic tanks, leaking sewer lines, land application of sludge and partially treated waste water. Sewage itself is a complex mixture and can contain many types of contaminants. The greatest threats posed to water resources arise from contamination by bacteria, nitrates, metals, trace quantities of toxic materials, and salts. Seepage overflow into drinking water sources can cause disease from the ingestion of microorganisms such as E coli,Giardia, Cryptosporidium, Hepatitis A, and helminths.

Raw Sewage Overflow
Storm water systems in urban areas are sometimes combined with sanitary sewer systems en route to sewage treatment plants. Excessive storm water can cause this joint system to overflow. In this event, excess flow will be directed into waterways untreated, resulting in sewage contamination. According to the EPA approximately 20% of the population is served by combined systems (1). Forty-six percent of the population is served by separate systems.Urban runoff is usually collected by a separate storm sewer system and discharged directly into waterways. Combined systems are cheaper, but the potential to harm health is higher. Some systems have diversions to accommodate heavy flow.

Septic Tanks
Septic tanks contaminate 1% of the nations usable aquifers (2). Septic tanks are enclosures that store and process wastes where no sewer system exists, such as in rural areas or on boats. Treatment of waste in septic tanks occurs by bacterialdecomposition. The resulting material is called sludge. Large portions of the population are still served by septic systems as opposed to public waste treatment facilities. Contamination of water from septic tanks occurs under various conditions:

Poor placement of septic leachfieldscan feed partially treated waste water into a drinking water source. Leachfields are part of the septic system for land based tanks and include an area where waste water percolates through soil as part of the treatment process.

Badly constructed percolation systems may allow water to escape without proper treatment.

System failure can result in clogging and overflow to land or surface water.

High density placement of tanks, as in suburban areas, can result in regions containing very high concentrations of waste water. This water may seep to the land surface, run-off into surface water or flow directly into the water table.

There are also site specific environmental factors around the tank and leachfield such as soil properties, water table location, subsurface geology, climate, and vegetation which may affect the quality and quantity of released waste water(1).

For information on maintaining your septic system, link to this article from Rhode Island University, Dept. of Natural Resources (3): Maintaining Your Septic System

Leakage from Sewer Lines
Effluent that leaks from sewer lines is generally untreated raw sewage. It may contain industrial waste chemicals. When leaking sewer lines are located deep underground below the biologically active portion of the soil, the sewage can enter groundwater directly. This can result in the introduction of chlorides, microorganisms, organics, trace metals and other chemicals that may cause disease and foul tastes or odors in drinking water (4).

Sewer leaks can occur from tree root invasion, soil slippage, seismic activity, loss of foundation due to washout, flooding and sewage back up, among other events (1). High pressure systems will push leaks to the soil surface where they can be easily detected by sight or odor. Systematic inspection of sewer lines, exclusion of hazardous waste, and adherence to modern construction and maintenance specifications are necessary preventative measures for protection of groundwater sources from sewer leaks(5).