Date. Class. Review and Reinforce. Radioactive Dating. Understanding Main Ideas. Study the diagram below. Then use a separate sheet of paper to answer the. Sep 7, 1Department of Marine Chemistry and Geochemistry, . activity of the ocean water that exchanges with the study area, zwc is the negligible radioactive decay (t1/2 = y) was then applied to the radium .. groundwater/pore water may not be overly enriched in nutrients, SGD may enhance the flux of. Date. Class. SECTION REVIEW AND REINFORCE. Ocean Water Chemistry. # Understanding Main Ideas. Complete the following table. The Water .
Sorption affects the concentration of dissolved organic compounds as well as dissolved metals in ground water. Two different phenomenon create the potential for sorption. First, organic molecules in aqueous solution are attracted by the net negative surface charge on humus solid organic matter.
Second, sorption occurs when electrostatic forces binding dissolved organic molecules to organic solids are stronger than the forces holding organic molecules in water Sposito, The distribution of organic molecules between water and organic matter is an equilibrium process and the amount sorbed becomes strongly a function of the amount of organic matter present.
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Organic compounds that react with soil organics in rural areas come mostly from agricultural chemicals and their degradation products. In urban areas, organic compounds that can react with soil organics include lawn-care products, oil and grease, and household chemicals. Soil humus may effectively arrest movement of these compounds into the subsurface. However, some compounds may become complexed with dissolved organic compounds, facilitating their movement into and through aquifers.
A general rule is that the more soluble in water an organic compound is, the smaller is its tendency to sorb onto organic or other substances Sposito, In summary, sorption is a reversible process that describes the affinity of dissolved species for the surfaces of solid materials, either organic or inorganic.
Because the process is reversible, metals or organics sorbed to surfaces can be des orbed if aqueous solution composition changes. This creates the possibility for addition of species to water through desorption that were not a component of the displacing aqueous solution fig. The mixing of fluids that can result from poor aquifer management could conceivably create such a situation.
This effect depends upon salinity here reported as ionic strength and also temperature and type of salt. Note that movement of saline fluids from deeper aquifers see section III could cause metals to leach from sediments; data from Long and Angino Both shales are from Kansas.
Ion Exchange An important mechanism of nonconservative behavior of ions in solution is ion exchange. The process of ion exchange requires a so1id phase that has a charge deficiency is negatively charged for exchange of cations, or a charge excess is positively charged for exchange of anions. Most solids that are important ion exchangers affect cations in solution, although anion exchange can occur in some circumstances Drever,p. The clay-mineral lattice then acquires a permanent negative charge that is not affected by the pH of the surrounding solution.
The negative charge attracts cations into specific sites within the layered structure of the clay mineral. The layers containing the exchangeable ions also contain water molecules, thus providing relatively easy ingress and egress for the exchanging ions. Capacity for exchange is determined empirically, by measuring the uptake and release of ammonium ions from a sediment sample. Different clay minerals have different ion exchange capacities.
Ion exchange is an equilibrium process, in which the exchange of one ion for another onto a solid material is described by a constant.
As with other equilibrium processes, the constant represents the ratio of the products of the chemical reaction to the reactants. A typical ion exchange reaction is: A more complicated situation exists for exchange of ions that have difference valences: Constants derived from experiments producing ion exchange are highly sediment and ion specific and are difficult to generalize. It is well established, however, that solids exhibit a preference for ions that is charge- and size-dependent.
The order of cation affinity for exchange is known as the Hofmeister series Stumm and Morgan,and is written from left to right in order of decreasing preference of the solid for the ion and so increasing likelihood of the ion residing in the solution: A generalized list of ion exchange affinities at approximately the same total concentrations could be as follows Domenico and Schwartz,again in order of most strongly preferred on the left to least preferred on the right: The process of ion exchange is completely reversible and occurs relatively rapidly as long as water and dissolved ions can pass freely through the sediment.
An important caveat is that clay minerals, the best ion exchangers, make up the largest portion of low-permeability materials mud layers and shales. The equilibration of low-permeability material with water recharging adjacent high-permeability materials is slow simply because of the time it takes the recharging water to penetrate the low-permeability material.
Thus, mud or shale provides a reservoir of exchangeable ions that can influence the chemistry of water within an aquifer over a long period of time. Recent work from two independent studies demonstrates the long time required for equilibration of marine shales with recharging fresher-than-seawater water Appelo, ; Chu, In Kansas, the Dakota aquifer fig.
Chu used a computer model to simulate the patterns observed in the water chemistry of the Dakota aquifer. Cation-exchange processes, thought to be the most important type of reaction affecting the water chemistry in the Dakota, are apparently ongoing. The computer model showed that, using reasonable numerical values to represent the Dakota's chemical properties, equilibration of clay minerals with the freshwater recharging it is not yet complete, even though the marine or marginal-marine clays were deposited more than 65 million years ago fig.
In the Dakota sandstones, the chemical patterns seen on maps of the Dakota water chemistry evolve through time as incoming freshwater displaces resident water and ion exchange of different ions occurs Chu, The best approximation of the evolution of ion exchange is that calcium and magnesium first replace sodium on ion-exchange sites and later calcium replaces magnesium on ion-exchange sites. The lateral map-view sequence created fig.
This sequence indicates that the clays are still yielding ions from ion-exchange sites that were saturated with sodium when the clays were in contact with sea water see table 5. The identified zones represent different stages of completion of ion exchange in the Dakota aquifer; from Chu, The plot shows concentrations in excess of positive values or as a deficit from negative values concentrations expected from mixing with no chemical reaction of resident fluid and recharging fluid.
The pattern shows expected changes in water chemistry because of cation exchange; from Chu In summary, the conservative and nonconservative dissolved species discussed above usually make up the largest proportion of dissolved solids in potable ground water.
KGS--Bulletin Water Chemistry and Sustainable Yield
The chemical behavior of labile and refractory species, discussed in the next section, is important. Labile and Refractory Species in Ground Water Introduction Labile species typically found in ground water include those that undergo changes in redox potential and those that are volatile. Refractory species are those that mayor may not have the capacity to undergo transformations but do not undergo transformation under ordinary groundwater conditions at significant rates.
The following discussion focuses on labile species but is limited to those most commonly studied in potable ground water. Two fundamental kinds of transformations are discussed in the following sections, those in which organic compounds are transformed into other compounds and those in which there is a change in redox state of an element.
Transformation processes often facilitate removal of a compound from water through precipitation in a solid phase or through change into a gas phase. Of course, the redox change can work in the opposite direction, to add components to water. Both types of transformations are typically mediated by bacteria, meaning that conditions in the aquifer must be favorable for activity of the appropriate bacteria.
Pumping a well can alter conditions in the aquifer, potentially either inducing or inhibiting these transformations. The change occurs principally because of introduction and mixing of different kinds of water from above, below, or laterally within the aquifer see below, The Consequences of Mixing.
In addition, organic compounds vary tremendously in their solubility how much dissolves in water and volatility tendency to form a gas. Redox reactions are those that involve the transfer of electrons and thus are involved in oxidation loss of electrons or reduction gain of electronsresulting in a change in valence for the species. Some elements can lose Or gain as many as eight electrons and can exist in multiple oxidation states with different valences.
Elements behave differently depending upon oxidation state, in that they exist in different kinds of solids with different solubilities and molecules and affect animals differently upon ingestion. The redox state of a fluid see Boxed section 5. For this discussion of labile species in ground water, the focus is on a nutrient nitrateorganic compounds, and metals with multiple redox states.
Although there are others, these are discussed because they are the most common contaminants in ground water and most often limit its use. Introduction of one or more of these species because of poor management of an aquifer could render the ground water unacceptable for its intended use, and thus these species have the most impact on issues of safe yield. Nitrate Nitrate is one of the most common contaminants of ground water. Nitrate almost always enters aquifers from the land surface, and thus factors that accelerate downward movement of water can in turn accelerate entrance of nitrate to an aquifer.
Nitrate mayor may not persist in water, as discussed below. Transformations among nitrogen species are typically mediated by specific genera of bacteria and are often studied within the context of the nitrogen cycle see Boxed section 5. The ultimate source of nitrate is the atmosphere, which is mostly nitrogen gas N2.Page 1-Radioactive Decay-Hommocks Earth Science Department
Although bacteria transform small amounts of N2 to NO in root nodules on legumes, most naturally occurring nitrate comes from decay of organic material that contains small amounts of nitrogen relative to carbon, hydrogen, and oxygen Drever, Oxidation of animal waste to nitrate barnyards, feedlots, septic systems and nitrogen-based fertilizers in agricultural regions are the two most important sources of introduced nitrate in ground water.
The two principal pathways by which nitrate is reduced are 1 through nitrous oxide species to nitrogen gas N2; called denitrification and through nitrite NO to NH called DNRA, dissimilatory nitrate reduction to ammonium; Korom, ; Smith et al.
Denitrification can be accomplished by bacteria using organic carbon as a source for electrons to reduce the nitrate heterotrophic bacteria or by using another source of electrons autotrophic bacteriaas shown by the following two equations: Absolute controls on rates of denitrification are not well established Korom,but an excess amount of easily oxidized material in an aquifer such as plant debris can reduce even high supply rates of nitrate to nitrogen gases and prevent contamination of an aquifer Simpkins and Parkin, Poor aquifer management can create a nitrate problem in ground water.
Much of fertilizer nitrate is used by plants or denitrified in the soil zone. Rapid passage of recharge water through the soil zone and bypass of the soil zone through flow in fractures can reduce the effectiveness of plant use, denitrification, and DNRA in minimizing nitrate content of recharge water. Issues associated with vertical movement of ground water and lateral movement as induced by pumping are discussed later in this chapter see Inter-aquifer Ground-Water Flow.
The Nitrogen Cycle Nutrients such as nitrogen are cycled through biological materials, the atmosphere, soils, and water in a continuous loop. The cycle, in a natural world, completely accounts for all available nitrogen, and none of the various parts of the cycle gain or lose nitrogen except as changes in climate affect the transformation processes.
In the natural cycle, rain and snow precipitation containing nitrate and ammonium fall on the earth. If the rain soaks into the soil to become part of the soil water, the nitrate and ammonium The nitrate and ammonium in the plants Ammonium that is adsorbed Nitrogen in the atmosphere Other sources of nitrogen, from human activities, are fertilizer and animal waste such as sewage, barnyard waste, and feedlot waste.
Nitrogen from these sources is subject to the natural processes described above, but overloading of nitrogen in an area can result in natural processes unable to transform the nitrogen species as quickly as they are supplied, with the result that ground water typically becomes enriched in nitrate.
The early stages of the fission chain reaction supply enough heat and compression to start deuterium-tritium fusion, then both fission and fusion proceed in parallel, the fission assisting the fusion by continuing heating and compression, and the fusion assisting the fission with highly energetic As the fission fuel depletes and also explodes outward, it falls below the density needed to stay critical by itself, but the fusion neutrons make the fission process progress faster and continue longer than it would without boosting.
Increased yield comes overwhelmingly from the increase in fission.
The energy released by the fusion itself is much smaller because the amount of fusion fuel is so much smaller. The effects of boosting include: Furthermore its decay producthelium-3, absorbs neutrons if exposed to the ones emitted by nuclear fission. This potentially offsets or reverses the intended effect of the tritium, which was to generate many free neutrons, if too much helium-3 has accumulated from the decay of tritium. Therefore, it is necessary to replenish tritium in boosted bombs periodically.
One mole of deuterium-tritium gas would contain about 3. Tritium in hydrogen bomb secondaries[ edit ] See also: Nuclear weapon design Since tritium undergoes radioactive decay, and is also difficult to confine physically, the much larger secondary charge of heavy hydrogen isotopes needed in a true hydrogen bomb uses solid lithium deuteride as its source of deuterium and tritium, producing the tritium in situ during secondary ignition.
The neutrons released from the fission of the sparkplug split lithium-6 into tritium and helium-4, while lithium-7 is split into helium-4, tritium, and one neutron. Therefore, the fusion stage breeds its own tritium as the device detonates. In the extreme heat and pressure of the explosion, some of the tritium is then forced into fusion with deuterium, and that reaction releases even more neutrons.
Since this fusion process requires an extremely high temperature for ignition, and it produces fewer and less energetic neutrons only fission, deuterium-tritium fusion, and 7 3Li splitting are net neutron producerslithium deuteride is not used in boosted bombs, but rather for multi-stage hydrogen bombs. Controlled nuclear fusion[ edit ] Tritium is an important fuel for controlled nuclear fusion in both magnetic confinement and inertial confinement fusion reactor designs.
The deuterium-tritium reaction is favorable since it has the largest fusion cross-section about 5. Analytical chemistry[ edit ] Tritium is sometimes used as a radiolabel.
It has the advantage that almost all organic chemicals contain hydrogen, making it easy to find a place to put tritium on the molecule under investigation. It has the disadvantage of producing a comparatively weak signal.