Lake Water Quality Measurements and Monitoring

The big lakes in the Weilheim region such as Lake Ammersee, Walchensee, Kochelsee and Staffelsee as well as the rivers Isar, Ammer and Loisach and other, lesser known bodies of water, have been regularly examined for many years by the Weilheim Water Management Office. Today, most bodies of water in the foothills of the Alps are typically rated from good to excellent. This success is because of comprehensive, corrective actions taken during the wastewater treatment process over the past decades. Time and money invested by the Water Management Office to make regular water quality measurements over the years has proven to be successful and helped achieve the high ratings.

Photo 1: Lake water quality measurements and monitoring in an ice hole.

Photo 2: Lake water quality measurements and monitoring using a depth probe.

How do we measure the success, and what water quality measurements are taken? The measurements concerning the water quality of lakes and streams are carried out according to regulations standard across Bavaria, starting with samples taken in-the-field. In streams, samples are most frequently taken from bridges with the aid of a special ladle device. In lakes, the deepest spot is usually the ideal location for testing. Lake Walchensee is 620 feet/189 meters, Lake Ammersee is 266 feet/81 meters. There, water samples are taken at defined depths with the aid of a ladle device (see photo 1). In the case of Lake Walchensee samples were taken at 0, 2, 4, 6, 8, 10, 12, 16, 20, 30, 50, 75, 100, 125, 150, 175 m, and 1 m above the bottom of the lake. Samples are taken throughout the year including the winter providing the lake ice is stable and safe for passing, which is regularly the case at Lake Eibsee. Therefore, the measuring devices must function and be reliable even at temperatures below zero.

First, the most important in-the-field physical and chemical parameters are determined immediately using handheld water quality instruments, such as the Multi 3500i: Oxygen concentration and oxygen saturation, temperature, conductivity and pH value.

The water samples are bottled and transported in coolers back to the lab for same day processing. First the samples are filtrated. From the filtrate, the nutrients, phosphorus, nitrogen (ammonium-N, nitrate-N, nitrite-N), silica (an important nutrient for certain plankton algae) are determined using photometric methods. In many bodies of water, the nutrient concentrations are at very low levels resulting from the cleaner effluent discharge requirements of most wastewater treatment plants; therefore, the measurement methods must be very sensitive to measure such low levels.

The Weilheim Water Management Office specializes in analyzing clean bodies of water by using a high throughput analyzer that measures a high number of samples reliably and at very low concentrations. The total phosphorus concentrations in the Lake Ammersee, for example, are commonly under 10 ug/l, and in Lake Walchensee under 5 ug/l.

With lakes, it is necessary to measure the physical and chemical parameters in-the-field at recorded depths by using a depth probe for more accuracy than that of a collected sample transported back to the lab. Especially strong temperature gradients occur in lakes during the stratification phase in summer. By using a depth probe, water quality measurements can be recorded true to depth. The Weilheim Water Management Office uses various water quality instruments with up to 328 feet/100 meter cable length (see photo 2). Big changes of the oxygen concentration within the depth profile can also be recorded without error. An example of these oxygen profiles from the Lake Ammersee (depth range 0-66 feet/0-20 meters) and from the Lake Dietlhofersee, which is only 62 feet/19 meters deep, is shown in figure 1. A current depth profile of temperature, pH value and oxygen concentration from the Lake Ammersee is given in figure 2.

Apart from the physical and chemical parameters and the nutrient concentration determination, plankton algae examination is also critical for water quality analysis. The concentration and type composition of algae is characteristic for the trophic level of a lake. For this, the chlorophyll concentration is measured in the collected samples, and the algae are analyzed with microscopic methods. These long-term examinations allow for early intervention in the case of negative events, and provide data to track improvements over time in the bodies of water.

Summary
Apart from water sampling and measurements in the laboratory, direct water quality measurement in-the-field is an important factor for monitoring the water quality. The direct recording of depth profiles provide more exact measured values compared to the less convenient sampling method, which is essential for the determination of the chemical parameters.

New water quality measuring instruments optimized for in-the-field use provide measured values with the highest accuracy and resolution. The data logger and serial interface protect the measured values against "human error" under difficult conditions. For example, multiparameter probes reduce the number of cables, thus making the recording of depth profiles much easier and accurate.

Multiparameter Water Quality Measurements
Normally, handheld water quality meters are used in-the-field. The Weilheim Water Management Office staff has had great success with water quality meters by WTW such as the ProfiLine series instruments. The new WTW multiparameter handheld meter, Multi 3500i is particularly suitable for water quality monitoring in bodies of water. With the simplicity of using a single water quality meter in combination with the corresponding standard measurement probes, the critical water quality parameters can be determined in-the-field and at virtually every temperature. Data collection according to GLP is enabled by a data logger with real time clock and date. When using the MPP 350 multiparameter probe and the Multi 3500i handheld multiparameter meter, depth profiles can be recorded both in lakes and bore holes while measuring oxygen, pH, conductivity and temperature simultaneously.