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Where Does My Tap Water Come From?

Where Does My Tap Water Come From?


We write a lot about water–how it smells, how it tastes, and primarily, what’s in it.

For those of you who test water with us, we write about how your water quality connects with your health. We’ve discussed (many times over) why water quality matters, but until now, we’ve not explained where our tap water come from in the first place…

In this article, we cover the basics about where your tap water comes from; whether you get water from a private well, a water system, or a rain barrel.

Surface water

Surface water is water that–put simply–you can see above ground. Think lakes, rivers, streams, and ponds. Surface water makes up the about ⅔ of drinking water supply. In addition to being the most prevalent water source for public supply, most people (86% of US population in 2010) are serviced by a water system.

Surface water Concerns

Surface water used for drinking purposes must undergo water treatment because it is subject to contamination from a wide range of naturally-occurring and human-inflicted activities:

  • Wastewater discharges
  • Stormwater runoff
  • Blue-green algal blooms
  • Bacteria (e.g. from algal blooms; poor sanitation conditions)
  • Agricultural and landscaping chemicals like herbicides and pesticides
  • Chemical spills
  • Mining & drilling for oil and gas
  • Traffic (e.g. brake fluids, construction sites, road maintenance chemicals)

The biggest source of surface water contamination in the U.S. is stormwater runoff. When it rains (stormwater) or when people excessively water their lawns and gardens (dry runoff), water flows over paved surfaces and picks up chemicals, trash, and other contaminants from roads.

This leads to increases in pollution and decreases in groundwater percolation (i.e. less water supply). 


Groundwater is a critical water source–making up 30.1 percent of global freshwater resources. Groundwater is water that seeps from surface waters fed by lakes, rivers, and precipitation.

This seepage leads to underground flows into and out of aquifers. Aquifers are porous rocks that store groundwater in large volumes. The water table marks the top of groundwater flow.

Private well owners, municipalities, and utilities all draw water from aquifers. Well owners typically use groundwater exclusively, whereas many utilities blend groundwater with other sources. Roughly 38% of U.S. drinking water from wells and utilities are from groundwater, whereas nearly 100% of self-supplied water (like private wells) comes from groundwater.

Groundwater Concerns

Like surface waters, groundwater sources are subject to contamination from a wide range of naturally-occurring and human-inflicted activities:

  • Natural occurrence of heavy metals (e.g. Arsenic, Chromium)
  • Bacteria (e.g. from poorly maintained septic tanks or iron-reducing bacteria)
  • Agricultural and landscaping chemicals like herbicides and pesticides
  • Landfill leakage
  • Chemical spills
  • Mining & drilling for oil and gas
  • Traffic (e.g. brake fluids, construction sites, road maintenance chemicals)

All of these activities lead to source water contamination. If you’re a private well owner, it’s up to you to test and manage the influx of these pollutants. We offer specialized testing for several known-groundwater concerns, which you can add to any of our Tap Score tests. If you are concerned about a specific contaminant and do not see what you’re looking for—let us know here.

Seawater & Rainwater

Rainwater–collected on rooftops or in barrels–and seawater treated by desalination–are two additional, but less common, sources of drinking water.


Desalination is the process of collecting very saline water and removing salts and minerals to produce freshwater. Usually, sea water is the source for desalination, but very “brackish” or saline groundwater can also undergo desalination. Fresh water has 1,000 parts per million (or 1 milligram of salt per liter of water) whereas seawater contains over 35,000 ppm of salts. Desalination technologies range from distillation to reverse osmosis.

Reverse Osmosis schematic. Source: SimpleLab. 

Desalination has been particular popular in dry, arid regions like California and Israel. It is also a popular at-home treatment technology for well-owners with particularly saline or contaminated groundwater.

Rain Barrels and Stormwater Harvesting

Collecting rainwater is a long standing technology that people have used for centuries: collect rain and store it. There are many DIY options for collecting rainwater at home, to water your garden or even go “off-the-grid” for your water supply (warning: this requires complying with state laws!).

Cities like Los Angeles are attempting to capture rainwater and stormwater runoff (which we noted is a big polluter for surface water above!) on massive scales. Cities use “spreading grounds” to collect and treat rain and stormwater runoff before letting it percolate back to groundwater, which eventually becomes a drinking water source.

Source: LADWP.

Rainwater Concerns

Airborne pollutants from particulate matter to organic pollutants can fall out of the atmosphere in water droplets. Additionally, the materials you use to collect and store rainwater can impact its quality. It is essential to test rainwater used for drinking water so that you can install proper treatment.

Tap Score Water Testing

How can I protect my source water?

Freshwater is a precious resource–and unfortunately a laundry list of human activities and industries leads to heavily polluted source water for our wells and water systems.

Ideally, we should be working hard to protect our source waters as individuals and communities. Regular maintenance of your septic system and minimal use of fertilizers and pesticides to avoid stormwater runoff are a few simple things you can do to protect your source waters.

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About The Author


Serving as the Chief Science Officer at SimpleLab, Jess Goddard spearheads the scientific program at Tap Score, overseeing all analytical products and services. With a Ph.D. in water resources and a Master's in environmental engineering from UC Berkeley, Jess brings a wealth of expertise to the team. Her leadership ensures the highest standards in our scientific endeavors, contributing to the excellence that defines SimpleLab and Tap Score. When away from her desk, Jess enjoys reading and being outside.
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