Introduction
On August 27, 2009 the State-EPA Nutrient Innovations Task Group issued an urgent call to action to EPA Administrator Lisa Jackson. The task group studied documented levels of excess nutrients in our nations waterways. Current, and past, efforts to control these pollutants have been undertaken on the national and statewide scale. Efforts to date have been predominately "regulation at the pipe" and have not addressed the non point sources that are the root cause of elevated concentrations of nutrients in ground and surface waters. Nutrient pollution significantly affects all of us. It affects our drinking water, our recreational water, such as beaches and rivers and lakes, and it affects, and even kills, aquatic life. None of us want a bunch of dead rivers, but what can we do?
What are Nutrients?
Nutrients are elements that are essential to life. Being essential to life they are also essential components of the molecules that make up living tissue. Decaying organic matter and human and animal waste are significant sources of nutrient pollution in ground and surface water. The chemicals we manufacture and use in day-to-day life may also contain nutrients, as well as the fertilizers that we spread on our yards and farms. It is the broad application of fertilizers, the animal feeding lots, the widespread use of septic tanks, and runoff water from rainfall to school kid carwashes that are responsible for the immeasurable depths of nutrients that find their way into the watershed. These are the non-point sources that can not be easily regulated.
For 30 years the Clean Water Act has regulated industrial and municipal effluents. Limits are getting lower and lower. It is easy to take a sample at the end of a pipe. It's easy to know where the sample comes from, and who is responsible if pollutants are too high. Unfortunately, 30 years of regulation of industry has not significantly reduced the problem. Obviously, there are other sources.
And the sources are us. Our everyday activities of washing our cars, fertilizing and watering our lawnns, irrigating farms, and even desiring green golf courses. The essential nutrients that are in the fertilizers and detergents are entering the groundwater, not by an industrial effluent but down the gutter and into the storm drain. Rainfall is washing nutrients into creeks and rivers and carrying them into basins such as the Gulf of Mexico or Chesapeake Bay. Hypoxic zones, also known as dead zones, are forming where excess nutrients are deposited.
The excess nutrients cause a rapid growth of algae. The algae grow so quickly that they cut off their own light and die. This is a normal, geologic process, occurring more quickly than normal. As the algae dies it decays and the decay process consumes oxygen. Without oxygen aquatic life can not breathe.
What is Nutrient Pollution?
Nitrogen and Phosphorus are the principal elements reported to when discussing nutrient pollution. Other essential elements, such as carbon, silica, and sulfur are not included in this discussion. Organic nitrogen and organic phosphorus are associated with the organic matter we measure and report as TOC. Nitrogen and phosphorus are essential elements in DNA, RNA, and nitrogen is a major component of protein and urine. Nitrogen and phosphorus occurs as both water-soluble and water insoluble species. Unfortunately, both soluble and insoluble become bioavailable. Looking at this table, we see that Nitrogen and Phosphorus are somewhat similar chemically, for instance, the plus 5 ions known as nitrate and phosphate are very stable and highly water-soluble.
Nitrogen
Nitrogen is an essential nutrient for plant growth ranking only behind carbon, hydrogen and oxygen in total quantity needed. The nitrogen gas making up about 80% of the atmosphere is large inert and unavailable to life directly. Lightning will convert small amounts of nitrogen to nitrate. Nitrogen in fertilizers comes from the chemical reaction between nitrogen and hydrogen gas to form ammonia. The ammonia can then be oxidized to form nitrate. Fertilizers will often contain ammonia, nitrate, and / or urea as the sources of nitrogen. Organic matter, referred to as humic matter, manure, mulch, etcetera contains about 5% Nitrogen. This nitrogen is slowly converted to ammonia by bacteria and is always oxidized to nitrite then nitrate. During a storm event, or during erosion the nitrate in soil is easily leached into the runoff.
Dissolved inorganic nitrogen including nitrate, nitrite, and ammonia. Dissolved organic nitrogen including water-soluble proteins, amines, amides, and so forth. Basically declined organic life and some man made chemicals that are dissolved in water. Total Organic nitrogen is the sum of dissolved organic nitrogen and particulate organic nitrogen. Particulate organic nitrogen is, well, the insoluble organic compounds, or organic matter, in the water. Total Dissolved Nitrogen is the dissolved organic nitrogen plus the dissolved inorganic nitrogen, and Total nitrogen includes all of the above. Notice, there is REALLY no such thing as total inorganic nitrogen. This is because inorganic nitrogen compounds are all water-soluble.
Chemical Analysis of Nitrogen Compounds
Total dissolved nitrogen, or the result you get when analyzing total nitrogen on a filtered sample complexes of dissolved organic nitrogen, nitrate, nitrite and ammonia. Methods used to determine TDN typically rely on an alkaline persulfate digestion that converts all of the nitrogen present to nitrate and then the nitrate is determined colorimetrically. Analyzing inorganic nitrogen alone will only recover about 30 – 40% of the total dissolved nitrogen in the natural environment. Sewage treatment and industrial plant effluents, on the other hand, are predominately inorganic nitrogen (nitrate) since the treatment process is designed to completely oxidize dissolved organic nitrogen and ammonia to nitrate.
Total nitrogen, or the result you get when analyzing total nitrogen on a non-filtered sample complexes of dissolved organic nitrogen, nitrate, nitrite and ammonia plus particulate nitrogen. Since all inorganic nitrogen compounds are soluble, particulate nitrogen is almost entirely organic nitrogen, or PON. Methods used to determine TN can rely on the same alkaline persulfate digestion used to determine TDN with the exception that samples are not filtered. Since samples are not filtered, the automated version is not applicable if there is a significant amount of sediment (or solids) in the sample. In other words, the automated inline digestion methods measure TDN and are only applicable if TDN and TN are essentially equivalent (there is no particulate organic nitrogen).
The DIN fraction, measured to calculate the Total Organic Nitrogen content can be, and should be, analyzed on a filtered sample. Remember that there is no such thing as Total Inorganic Nitrogen since it is equal to Dissolved Inorganic Nitrogen. Another infamous parameter in the total nitrogen world is, of course, TKN (Total Kjeldahl Nitrogen). TKN, as routinely used, does not measure nitrate or nitrite. It is essentially a measure of organic nitrogen plus ammonia nitrogen.
TKN is the classical, if you will, analysis for total organic nitrogen. TKN has its roots in food and feed analysis as a way to quantify the amount of protein. TKN has been extrapolated to environmental analysis and is the regulated parameter for Total Nitrogen. Unless steps are taken to include nitrate and nitrite the regular TKN method does not measure it. TKN essentially measures organic nitrogen and ammonia. TKN is sufficient in POTW or municipal influences because these samples rarely have nitrate in them anyway. Recall the nitrogen reactions. An influent to a POTW will contain particulate nitrogen, dissolved organic nitrogen, and ammonia. The particulate nitrogen mostly settles as sludge removing it from the equation. Organic nitrogen and ammonia are oxidized to nitrate during the treatment process. The TKN digestion boils the sample in concentrated sulfuric acid in the presence of a metallic catalyst to speed the reaction. Potassium sulfate is added to raise the boiling point to about 380 C. The digestion will not completely recover all organic nitrogen compounds making the TKN result actually less than or equal to the TN determined by alkaline persulfate oxidation.
According to the EPA definition, Total Nitrogen equals TKN plus nitrate plus nitrite.
Phosphorus
Phosphorus is an essential nutrient found in living organisms as part of DNA among other important molecules. Phosphorus is always found in nature bound to other atoms and usually as the inorganic phosphate. It is phosphate that is available to plants and used as a fertilizer. It is phosphate that is consumed by algae and has the potential to cause algal blooms. Measurement of total phosphorus is important because it can, with time, convert to bioavailable soluble phosphate.
Dishwashing, laundry, and many hand detergents and / or soap contain phosphorus. The phosphorus content in detergent can be as high as 8.7%. Phosphate is a very effective way to improve soap quality, especially in waters that contain high amounts of calcium and magnesium. Unfortunately, the phosphates in these detergents find their way into the environment. In the 1970's green rivers and lakes were becoming common and phosphate usage in soaps began its eventual reduction. Although there is not a federal ban on phosphate is soap many states are taking action. For example, Washington State has limited the amount of phosphate in dishwasher detergent to 0.5%
Chemical Analysis of Phosphorus
Phosphorus can exist in water in different forms. The standard method for phosphorus is meant to measure only phosphate, also called reactive phosphate. This is because what is actually measured is defined by the molybdate reaction itself. Phosphorus species are distinguished from each other empirically by filtration, and then a series of digestions that selectively convert phosphorus to phosphate. After the digestion phosphate is measured. Thus, to analyze organic phosphorus only, one digests for total phosphorus in one sample aliquot, and hydrolysable phosphorus in another aliquot. Reactive phosphate is then determined in each digest and Organic Phosphorus is calculated by difference.
Total Phosphate is water-soluble. Instrumental methods require that samples be filtered, or the turbidity and / or solids will interfere. There is certainly to measure total phosphate you must filter the sample. Since total reactive phosphorus is equal to filterable reactive phosphorus the results are the same. Remember though, that for best results, filtration should be in the field. The portion for the total analysis is not filtered. If the particulates (TSS or SS) are high, then continuous flow methods should not be used for the analysis of Total Phosphorus. Total phosphorus is batch digested that converts all phosphorus compounds to phosphate. The digest is filtered and phosphate is then measured, usually by molybdenum blue.
The continuous flow method for TDP utilizes 254 nm UV irradiation to assist in the digestion of organic material. Since continuous flow methods can not be used to adequately digest samples containing high amounts of solids and continuous digestion should not be used unless the total phosphorus is essentially equal to the dissolved phosphorus.
TKN is the classical digestion for total organic nitrogen. The TKN digest can also be used to analyze for total phosphorus. The advantage in this is not added recovery, for the acid persulfate quantitatively recovers all of the total phosphorus. The only advantage in the TKP is the ability for the laboratory to test for TKN and TP in a single digest. The perceived benefit is time and labor savings. Because of the higher acid and salt concentration of the resulting TKP digest compared to persulfate digests, TKP detection limits are typically higher that limits found by persulfate. The other advantage to TKP is, of course, that it is EPA approved.
All of the colorimetric methods used for the determination of phosphate by molybdenum blue are highly dependent upon final acid concentration and the amount of molybdate. Attempts made at analyzing total phosphorus digests without careful attention to acid concentration are often not successful. After the digestion is complete, you are measuring orthophosphate. Any method used to measure phosphate can be used. The problem with extending a total P determination to Ion Chromatography, for instance, comes from the excess sulfate ion introduced in the digestion. Early methods utilizing persulfate in sulfuric acid neutralized the sample and then analyzed orthophosphate. Some continuous flow methods measure the phosphate in the acid solution adjusting the acid in the reagent accordingly. The TKP digestions have really only been tested using a mercury catalyst, and excess chloride must be added to prevent interference from the mercury. Like TKN, the blue color of a copper catalyst may interfere with the method. The auto dialysis method helps to control the final acid concentration and some of the residual color introduced by the copper. Dialysis is essentially an online dilution leading to a higher detection limit than the cleaner persulfate digest. To sum up, for lower total phosphorus numbers (say below 0.1 ppm) it is recommended to use the acid persulfate digestion because the matrix will be easier to control and work with. For concentrations above 0.1 ppm, the TKP digestion is adequate.
Conclusion
The clean water act was signed amid stories of massive fish kills and rivers catching on fire. We all know that these things rarely happen in the US anymore. We have monitored sewage treatment effluents and in many cases applied numerical volumes on the amount of nutrients that can be discharged. We still have a ways to go.
Our chemical analysis methods were written for highly polluted water. The methods we use were not validated at the lower concentration levels we need to measure today. Many of the conditions of these methods assume higher concentrations will be present, and the former rigid nature of the Clean Water Act rules did not let us change anything. Fortunately, in 2007 the EPA added Part 136.6 to the CFR allowing us to make configurations to our nutrient methods providing the modifications improve method performance. These modifications are going to allow us to monitor more water, with better accuracy and detection limits.
We have more to go, because regulation of point sources has not solved the problem. Much of the problem is the non point sources we call storm water run off and irrigation water. We need to watch what we do when we apply fertilizers and watch what goes down our storm drains. But more importantly, what we need is increased monitoring of our rivers and streams. The only way we can begin to solve our problems is to know exactly what they are. To understand this problem we need more data, and the data we need is accurate, low level quantitative analysis of nutrients.