The capture and reuse of runoff for irrigation is of vital importance to the sustainability and growth of the ornamental horticulture industry. This practice has brought up a number of important questions including: How does water quality in retention ponds differ from other sources? Does recycled water affect the performance of chlorination and pesticides?
Recycled water quality fluctuates dramatically over time. We have been continuously monitoring water quality in several retention ponds in comparison with that in an adjacent stream or relatively clean water resources 24 hours a day and 7 days a week since 2006. The extent of water quality fluctuation in retention ponds was dramatic. Water pH ranged from 6.4 to 10.3, averaging 8.4. In contrast, the water pH in an adjacent stream was constantly below 7.0.
Water quality changes diurnally
The levels of pH and dissolved oxygen in water are closely related to photosynthesis activity in ponds. When the sun rises, algae and other photo synthetically active agents remove carbon dioxide, a weak acid, from water to make carbohydrate while releasing oxygen. Consequently, water pH and dissolved oxygen goes up. This process is expedited with rising temperature. Thus, temperature, pH and dissolved oxygen fluctuate almost simultaneously.
They all typically bottom around 6 a.m. and peak between 4-5 p.m. The range of fluctuation depends on the parameter, nursery location, nutrient load, and the time of year. The greatest diurnal fluctuation range for water pH across all ponds monitored so far was 3.5 with the lowest pH reading at 6.5 in the morning and the highest at 10.0 in the afternoon.
Water quality impacts
According to the current BMPs, the ideal water pH range for ornamental crops is from 6.5 to 7.0. Water pH above 7.0 could negatively affect crop quality or productivity. However, there is very limited data regarding what crops are most prone to pH stress, at what degree and by what mechanisms they may be impacted.
The greatest electrical conductivity (EC) of recycled water observed in this pond was well below the threshold level. Similar EC ranges were observed in other nursery retention ponds. EC could go up to 1,400 microsiemens/cm or even higher, which could be a concern in greenhouse settings. There is no research data regarding how dissolved oxygen and other parameters such as oxidation-reduction potential may affect crop health and productivity.
Water pH is known to affect the performance of chlorination, a commonly-used water treatment, and pesticide performance.
Water pH is of primary concern in recycled water quality management. Compared to natural water, recycled water in retention ponds is alkaline for the most of the year, especially during the growing season. Recycled water pH also fluctuates much more frequently over time and at greater extent diurnally. Irrigation managers need to test recycled water pH more frequently. It’s likely they’ll have to acidify water for crop quality and productivity as well as the optimum performance of chlorination and pesticide applications.
As a note of caution, pH must be taken at the time of water use as it could change substantially within a few hours. This is particularly true between 10 a.m. and 2 p.m. It is recommended to use a handheld digital pH meter instead of the colorimetric types for better accuracy.
Chuan Hong is a professor at the Virginia Tech, Hampton Roads Agricultural Research and Extension Center; email@example.com. This research originally appeared in the Virginia Nursery & Landscape Association newsletter.