I took a series of DO concentration readings at freshwater springs distributed over a large geographical area. Four weeks elapsed between the first measurement and the last. Each site was sampled only once. The DO concentration values of all readings are considerably lower than I expected.
All measurements were taken using a multi-parameter water quality meter from Hanna Instruments (HI-98194). Calibration was completed the day before the start of the sampling campaign.
There might be two reasons-
One, the freshwater is becoming eutrophic which is not prominent to see at present but causing the death of cyanobacteria and microalgae to die and in the process, dissolved oxygen gets consumed. Due to enrichment of essential nutrients like P, N, K, Ca, Mg etc., freshwater bodies develop intense microbial flora resulting eutrophication and sometimes such microbial flora does not show up at the water surface at a bubbling mat but causes reduction of the DO with gradual death of the microbial cells.
Second, may be due to some chemical addition to this water body is causing the reduction of DO in the process of chemical deposition with changed redox status. You can try to measure the redox potential (ORP) values to check if that's also decreasing or not.
Wim Kaijser Our readings range from 0 to 45.6 % DO (with an average of 21.4% DO).
I wouldn't think that groundwater would have much dissolved oxygen usually. If it had some when it entered the ground, I would suspect that chemical or biological processes would eat up the oxygen at some point in the path. From there, the groundwater isn't really mixing with the atmosphere. Though if it's flowing through soils or sediments with air-filled air space, the water may pick up some oxygen there. Am I off base?
Hi Giulio,
Do you have DO in mg/l too? Saturation (%) is a relative measure moderated by temperature, i.e. the amount of oxygen present as a % of the max possible amount at that temperature. The groundwater DO might be very similar, but site-specific factors like presence of primary producers, shading or sampling factors like time of day, distance from the upwelling etc. may be influencing temperature, and thus saturation.
Best
Kieran
Hi Giulio Biondi
There are many causative agent lead to decline DO in freshwater, one of them decrease plant in the aquatic environment, increase organic materials and fish overcrowding as well as elevate water temperature
Best regards
% are a bit of an difficult measure around-ish 80-120% is somehow "normal" for surface water, whereas 150-200% would indicate severe eutrophication due to primary producers. For fish, they have some arbitrary boundary of ~~8-10 mg L DO seems mostly used as "normal" unimpacted upstream areas. Yet, it always declines from upstream-downstream. Less turbulent waters, more primary consumers, increase in FPOM that consumes oxygen, etc.
In ground water it is "normal" to find around
Kieran Gething Yes, the meter provides a reading in ppm DO, which is equivalent to mg/L DO as far as I understand. I have verified this by using an online conversion tool and my values of % DO to estimate mg/L DO as a function of temperature: it all checks out.
The temperature readings are in the 11 ± 3 °C range, which is not too far from those measured by Schürch and Buckley (2002) from boreholes in chalk bedrock.
All taken into account, my readings are still low.
Yes, the increased organic elements is a genuine cause for the decline in DO in any water body but there is the effect of temperature as well. I have myself worked on a spring-fed lake wherein these factors mainly showed their impact.
Excess algal growth due high presence of phosphorus or nitrogen. The accumulation of undecomposed organic waste in the water could also cause low DO. DO concentration is affected by diffusion and aeration, photosynthesis, respiration and decomposition.
DO levels also fluctuate with temperature, salinity and pressure changes; the higher these parameters, the lower the DO
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