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Is Tap Water Okay for Hydroponics?

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Contaminated water collected in water bottles.

Water Quality Report—First Things First

The answer is dependent on your water chemistry. Tap water quality fluctuates greatly throughout a region, over the seasons, and can even vary from site to site. If you are on city water, contact your municipality and they can provide you with free water reports, usually these reports can be found online. If you are on well water, you will need to test the water yourself. This can be done through a local lab, with mail-in kits like these from SimpleLab™, or by working with Arvum Plant Labs.

Understanding the source water chemistry helps hydroponic growers decide whether or not using a water filtration system would be beneficial.

Water Contaminant FAQs

What is in tap water?

In addition to the naturally occurring minerals present in local surface water or groundwater supplies, all municipalities add chlorine and/or chloramines and fluoride for the purposes of human health. Water from well or spring sources are often high in minerals such as sodium, calcium, magnesium, sulfur, iron, and manganese. Although city water treated by municipalities must meet the minimum EPA standards, well water is not regulated and can also contain harmful contaminants like chromium, arsenic, or bacteria. While city or well water may meet drinking water standards, it may not be ideal for plants or hydroponics.

Why is chlorine in tap water?

While chlorine is a biocide added to water supplies to keep humans safe from harmful bacteria, it will also kill any added beneficial bacteria, fungi, and microorganisms for hydroponics. Therefore, a healthy organic or biohydro garden should be chlorine-free. Hydroponic gardeners that use or brew compost teas or bioextract solutions should remove chlorine from their source water. Learn more about how to dechlorinate your water by reading “Hydroponic Dechlorination for Stronger, Healthier Plants.”

What are chloramines?

Chloramines are an even stronger biocide that is a combination of chlorine and ammonia and is much more stable than chlorine so that it will not dissipate across long distribution systems. Because they are stronger than chlorine, they are not only toxic to beneficial bacteria, fungi, and micro-organisms, but are strong enough to kill fish and amphibians.

How do you remove chloramines?

Chloramines will not dissipate by bubbling or even by boiling off. They can only be removed by proper filtration such as through an activated carbon KDF filter.

What is water hardness?

Water hardness is made up of calcium and magnesium dissolved in water and is the key cause of most water problems in hydroponic facilities. High levels of hardness in untreated water lock out key nutrients to plants and also form scale on equipment and tubing. Calcium and magnesium are often the most abundant hardness minerals in tap water. The most typical form of these minerals in water is calcium carbonate and magnesium carbonate, and is often measured in grains per gallon (gpg). The molecules of these compounds found in untreated water are far too large and immobile to be absorbed efficiently by the root systems and transported to where the plant needs them.

For hydroponics, hardness is best removed by a reverse osmosis system which will reduce hardness by over 95%. While a softener will also remove hardness, it will substitute sodium for the hardness, making it difficult to grow plants with water that contains high levels of sodium.

Is there fluoride in tap water?

Fluoride is added to the majority of municipal water supplies across the US. You can check if your city is adding fluoride by viewing the water quality report published online.

Is fluoride bad for plants?

Fluoride in high concentrations can be harmful to plants and even humans. Thirty-four enzymes in plants are affected by fluoride, as well as seed germination. Enzyme additives will not do their job properly with fluoride in the water. While there are expensive specialized fluoride removal cartridges, it is best removed by a reverse osmosis system

Do volatile organic compounds (VOCs) affect plants?

Some VOCs are known or suspected carcinogens. Trace amounts of these can end up in the plant’s tissues, flowers, and fruits. VOCs can be removed by either a carbon filter or a reverse osmosis system.

How does iron or sulfur affect water and hydroponic equipment?

Water containing iron or sulfur may have a metallic taste and an offensive odor. Nutrient lockout and equipment staining can be the result of too much iron in the water. Additionally, iron can rapidly scale reverse osmosis membranes, requiring frequent replacement. A reverse osmosis system can remove sulfur and low levels of iron, but if iron concentrations are high, a backwashing iron filter may be needed.

How does bacteria affect groundwater quality?

Groundwater sources may be contaminated by animal and human waste and other pathogens. These toxic substances can cause multiple issues in reservoirs and nutrient mixes. E.coli can be dangerous for human consumption and compromise the quality of your end-product. A reverse osmosis system followed by a UV sterilizer is an effective way to remove bacteria without added chemicals.

How do nitrates affect plant growth?

Found in runoff from agriculture, animal yards, etc., these toxic substances contribute to over-nitrification and algae growth. High quantities of nitrates in untreated water can negatively affect the later stages of flowering, when cultivators typically try to limit nitrate levels in the growing medium. A reverse osmosis system will remove 95%+ of nitrates present in the feed water, giving hydroponic growers a consistent base from which they can control their nitrate levels during fertigation.

How does pH affect plant growth?

Water that has either too high or too low pH will not allow nutrients to be absorbed properly and can be corrosive to equipment. Adjusting the pH of nutrient solutions may be difficult due to fluctuations in source water mineral levels. While reverse osmosis water is typically acidic and unstable, it will make it much easier to control pH levels during fertigation with fewer minerals in the water.
Some cultivators are lucky enough to enjoy clean tap water that’s readily available for their hydroponic system. Even if you believe your geographic area has a "good" water source, not investigating the water quality and giving your plants less than pure water could choke their potential, and undermine an otherwise perfectly engineered grow.