In some cases the water is being derived from large limestone aquifers and in other cases the water is being derived from shale formations which have been the site of historic oil and natural gas drilling. In other cases, the wellheads are sited in karst areas which are frequently promulgated with sinkholes and caves that can act as conduit for surface contaminants like road salt to contaminate the aquifer.
Most greenhouse operators do not request that their irrigation water be tested for chloride and/or sodium. One new operator purchased a farm that had four sources of water on the property (spring, well, pond, and stream). At first the grower ignored the extension agent's recommendation to test the quality of the well water before initiating production. After observing cropping issues (as observed in the photo of the Altenanthera spp.) the grower opted to test his well water for "everything". The tests revealed that the EC of the well water was 2.37 m/S with chloride levels at 523 mg/l and sodium levels at 226 mg/l. While the EC level was high and had to be accounted for, the grower had two bigger problems to worry about. The upper limits for chloride and sodium in irrigation water are both 30 mg/l (as per the Penn State laboratory) and the levels of both were extremely elevated.
Subsequent testing and evaluation of the grower's pond and spring revealed that both of these water supplies also contained elevated levels of chloride and sodium which would render them "unusable" from an irrigation perspective. The use of a series of cisterns to collect rainwater to dilute the well water was considered impractical and the cost to treat the water through a reverse osmosis system was too high for the volume of water needed. As a result, the grower opted to connect to the public water supply rather than pump water with variable water quality from his stream which was located almost a half mile away.
In conclusion, please remember to annually evaluate the quality of your irrigation water even if the water source has been used safely for years. Poor quality water may impact your fertigation programs while reducing crop quality and crop yield (vegetables).
A Water Quality Toolkit for Greenhouse and Nursery Production
Water Quality Checklist for Greenhouse Growers
This article also appears on the eGrow Blog. Penn State Extension educators are regular contributors and Penn State Extension is a collaborating organization in e-Grow. e-GRO (Electronic Grower Resources Online) is a collaborative effort of floriculture specialists to create a new clearing house for alerts about disease, insect, environmental, physiological and nutritional disorders being observed in commercial greenhouses. Information is available about disorders, podcasts, and research. Bringing together some of the leading specialists from universities around the USA, e-GRO is a free resource and learning tool for anybody involved in greenhouse plant production.