A knowledge jump from high frequency monitoring

Transcription

1 Knowledge journal / Edition 1 / 2016 Polder water quality A knowledge jump from high frequency monitoring The water level in the Netherlands is mainly regulated by pumping stations. Pumping stations are traditionally also the location to monitor water quality. During the last decades, more and more pumping stations have been equipped with automatic switching systems and run predominantly during the night hours, while the water quality is still mostly measured during the day. According to research, this can have a substantial impact on reported trends in water quality. The De Blocq van Kuffeler pumping station is one of the four pumping stations that keep the Flevo polder dry. Deltares established a monitoring station in September 2014 in collaboration with the Regional Water Authority Zuiderzeeland, which measures the concentration of nitrate (NO3), total phosphorus (TP) and suspended sediments (SS) in the Lage Vaart, every five to ten minutes. It is the world's first high frequency monitoring station for water quality at a pumping station. The drainage area of the Lage Vaart is used mainly for agriculture. The dynamics of nitrate The nitrate concentration at pumping stations seems to be related to drainage water from the agricultural area in the polder. The measurements indicated a low nitrate concentration at the pumping station during the period from 1 October to mid November and from about mid April to the end of August. During these periods, the groundwater levels were lower than the tube drainage level. During intense rainfall events in the summer and prolonged wet periods in the winter, the groundwater level increases to up to or above the tube drainage level, and an increase in nitrate concentrations is measured at the pumping station about five days after a rainfall peak. It was already known that the nitrate concentration in the drainage water in the Flevopolder is relatively high (with concentrations measured between approximately 5 and 20 milligrams N per litre). As a result of the shrinkage cracks in the Flevopolder's clay soil, there is a rapid discharge of nitrate when the groundwater levels reach the level of the drainage tubes. Our measurements indicate that this drainage water controls the quality of surface water to a large extent under wet conditions. Effects of manure spreading In order to get better understand the sources and transport routes of nitrate, a survey was conducted to examine whether a relationship can be established between the high frequency concentration series of nitrate and the precipitation (hourly data), using time series analysis (a transfer function noise model). These results are shown in figure 1. The top graph shows the measurements and the modelled concentration time series, the lower figure shows the residuals (measurement minus model). The time series model indicates that 70 per cent of the dynamics in the nitrate concentration can be explained by a reaction to precipitation. The results indicated that the decline in the concentrations during dry periods were properly modelled. The wet periods show varying results: the increase in concentration was properly modelled in December; in January, the concentration was overestimated, while the concentration was underestimated in February/March.

2 Figure 1. High frequency measuring set for nitrate, precipitation (sum of the previous 24 hours) and the timeseries analysis model Differences between the modelled and measured concentration give additional insight into the nitrate dynamics in the Flevopolder. The overestimation in January is probably the result of dilution due to heavy rains in combination with a decrease of in the NO3 stock stored in the soil profile due to leaching with rain during previous month. This effect is not included in the transfer model. The underestimation and especially the peaks in measured concentration in February/March can be explained by manure application in that period. The Manure Law allows for spreading of manure on arable land from 1 February and 15 February on grassland. The first three weeks of February 2015 were dry. It is likely that manure was applied to land on a large scale during this period. On 20 February, it started raining heavily and a few days later, on 24 February, the nitrate concentration peaked at almost 10.5 milligrams N per litre. The measured nitrate concentration was also higher than the modelled nitrate concentration during the subsequent rain showers in late February and early March. Total phosphorus dynamics While it is quite simple to a indicate a dominant source for nitrate, the same can not be said for total phosphorus (TP). The concentration at the pumping station increases structurally during dry periods and is at its highest in the summer. This indicates the influence of phosphate rich groundwater and a later release of phosphate from the water bottom. Other sources of phosphorus are phosphate rich water from the Oostvaardersplassen and the sewage treatment plant at Almere, who discharge on the Lage Vaart. The nitrate rich agricultural water, which has a huge impact on the water quality during wet conditions, contains notable low TP concentrations (Figure 2). The precipitation on December led to a peak in the nitrate concentration of 5.5 milligrams N per litre on 17 December.

3 Figure 2. Dynamics in nitrate, total phosphorus concentration and suspended solids (as turbidity) after a wet period on 10, 11 and 12 December, with the precipitation and the pump regime Aside from the peak in TP concentration during the initial stage of pumping, the concentration on 17 December was less than on 11 and 12 December. Although it cannot be excluded that the drainage water from rural areas contains increased phosphorus concentrations, this cannot be seen at the pumping station. There may be leaching of phosphorus from agricultural soils to surface water during wet conditions, but it will not be transported directly to the pumping station. A part of the phosphate in the soil that will be leached to the surface water is bound to fine particles. In addition, dissolved phosphate will be attached to iron oxide particles in surface water that are formed in the ditch by seepage of ferrous groundwater. Sedimentation of these forms of particular phosphorus in the ditches and channels prevents rapid transport to the pumping station. Sedimented phosphorus can lead to a later release of phosphate from the bed sediments to the surface water in the summer, thus creating a 'delayed' release. The high ortho P concentrations during the summer indicate a late release of P from the bed sediment. Such conversion processes and the temporary storage of phosphate doesn't make it any easier to quickly determine the effects of agriculture practice and manure measures on phosphorus concentration in the surface water. The effects of pumping Previous research in Friesland has already indicated that nutrient concentrations could increase during pumping. We were able to better quantify this effect using the high frequency TP and SS measurements at the De Blocq van Kuffeler pumping station. The TP and SS concentrations increase suddenly when the pumping station is in operation (see Figure 3 for TP). The TP concentration increases 0.06 mg P per litre on average over a year when pumping with one pump; this becomes 0.13 mg P per litre when pumping with two pumps. The concentration of suspended sediments increased on average by 4.4 milligrams per litre when pumping with one pump, while this was 22 milligrams per litre when pumping with two pumps.

4 Figure 3. Change of the TP concentration during pumping This increase when pumping indicates that the change in flow rate of the water leads to resuspension of sediment bound P from the water bottom. The increase in TP concentration during pumping was much smaller in the surveyed polder than what was measured during wet periods in 'free drainage' areas in the Netherlands or beyond, where concentration increases were reported with a factor of 100 or more. Because the flow rate of the water in polders is limited to the pumping capacity of the pumping station, the risk of resuspension of large amounts of P is also limited. Polders therefore have a large capacity for storing P in the surface water system. As a result, the risk of releasing P from the water bottom at a later stage, and thus for higher concentrations in summer, is much greater for polders than for free drainage areas. The De Blocq van Kuffeler pumping station runs mostly in the evening and at night because power is then cheaper. Water quality samples are always taken during the day. This leads to an underestimation of the TP concentration of the water that was actually pumped water. This has implications when determining the loads from the 'Lage Afdeling' to the Markermeer and possibly also for trends in water quality. In 2008, the pumping station was converted from a manually operated, diesel engine powered pumping station, to a fully automatic, electric motor powered pumping station. It looks pretty much like this transition can be seen in the TP concentration time series (Figure 4). The time series from 2000 to 2015 indicate a declining concentration. The time series from 2000 to 2009 and from 2009 to 2015, however, indicate increasing concentrations. These differences are caused by the sudden reduction in TP concentration at the beginning of 2009, exactly the time that the pumping station was converted. Although it may not be entirely excluded that there are other causes for the sudden decrease in TP concentration, it is likely that this has to do with the change to pumping during the evening and at night.

5 Figure 4. TP concentrations time series, the LOWESS trend and slope In the past twenty years, a large number of pumping stations in the Netherlands were converted from diesel powered to fully automatic electric pumping stations, mainly operating during the evening and night hours while sampling takes place during the day. It is therefore conceivable that the change in pump regime on a regional and even national scale, may have an effect on reported trends in water quality of the actually pumped water. Sampling of water quality with grab samples during the daytime at pumping stations that run primarily at night, lead may to an underestimation of export loads of substances that have a high affinity for binding to sediments. A comprehensive article on the described research will shortly appear in the magazine Hydrology and Earth System Sciences. Bas van der Grift Joachim Rozemeijer Hans Peter Broers (TNO Geological Survey of the Netherlands) Wilbert Berendrecht (Berendrecht Consultancy) Michiel Oudendijk Zuiderzeeland Water Board Summary Water levels are regulated with pumping stations in around 60 percent of the Netherlands. They pump excess water onto the polders. The water quality is often measured at these pumping stations. Sampling of water quality during the daytime with grab samples at pumping stations that run primarily at night, leads to an underestimation of the pumped quantities of substances, like phosphate and suspended sediments. Concentrations can now be measured at high frequency using new measurement techniques, allowing one to get a much better picture of the origin of certain substances and the nature of the transport processes. High frequency measurements at the De Blocq van Kuffeler water pumping station in the Flevopolder have indicated that nitrate from agricultural areas is drained away relatively quickly in wet periods, but that

6 phosphate is actually retained much more in polder systems than in free drainage areas. Phosphate emissions occur, therefore, not predominantly in winter, but due to the later release from the bed sediments and by feeding from the ground water, rather in summer. Auteurs Bas van der Grift Joachim Rozemeijer Hans Peter Broers (TNO Geological Survey of the Netherlands)

7 Wilbert Berendrecht (Berendrecht Consultancy) Michiel Oudendijk (Zuiderzeeland Water Board)