Technical Summary
TS.3. Estimation of 131I Concentrations in Fresh Cows' Milk

 


The transfer of 131I from deposition on the ground to fresh cows' milk is relatively well documented. FigureTS.2 illustrates the parameters involved in that transfer. The time-integrated concentration of 131I in milk, IC, corresponding to an estimated deposition density on the ground, DG, on a given day (d) and in a given county was calculated as:

in which F* is the mass interception factor (m2 kg-1 (dry weight)), Te is the effective half-time of retention by the vegetation in days (d), PI is the pasture intake (kg (dry weight) d-1), and fm is the intake-to-milk transfer coefficient (d L-1). Each of these parameters will be discussed.


TS.3.1. Mass Interception Factor

The mass interception factor (F*) is defined as the fraction of 131I activity deposited on the ground which is intercepted by vegetation (F), divided by the standing crop biomass (Y). The mass interception factor depends on the meteorological conditions, on the characteristics of the depositing material and on the type and density of vegetation. Values of mass interception factors obtained in laboratory or field experiments conducted under dry or light spray conditions with a variety of radionuclides show a range of variation of 1 to 4 m2 kg-1 (dry) .

The mass interception factor is usually estimated as:

where the numerical value of a, the foliar interception coefficient, is equal to 2.8 m2 kg-1 (dry weight) for elemental iodine and small-size aerosols under dry or light spray conditions.

Figure TS.2. Transfer of 131I from deposition of the ground to fresh cows' milk.


There is evidence that the value of a decreases as the particle size increases and, therefore, that themass interception factor decreases as the particle size increases. In the case of atmospheric nuclear weapons tests, large-size particles fell out near the detonation site and smaller particles were deposited as the radioactive cloud moved further away. The variation of a (m2 kg-1, dry weight) as a function of distance from the NTS, X (km), was expressed as:

With the use of this expression, the value of a increases with distance from the NTS and is approximately equal to 2.8 m2 kg-1 (dry) for X = 1540 km. Beyond that distance, the value of a is taken to remain constant at 2.8 m2 kg-1 in order to remain consistent with the value obtained from equation TS.2 for elemental iodine and small-sized aerosols.

All of the laboratory and field experiments reported in the literature were conducted under dry or light spray conditions and do not, therefore, provide any information on the values to be expected in the case of moderate or heavy rainfall. On the basis of experimental studies on the initial retention of rainwater by vegetation, it is proposed that the variation of the mass interception factor as a function of the rainfall amount, R (mm), can be estimated by:

where a and b are empirical parameters, with values of 0.9 m2 kg-1 (dry weight) and 11 mm kg-1 (dry weight) m-2, respectively. This equation is being used in the report for daily rainfall amounts in excess of 5 mm.

Given the importance of the mass interception factor in the assessment of the 131I exposures, and the limited information on its value under conditions of moderate or heavy rainfall, a research program was designed to investigate the dependence of the mass interception factor on the nature and physico-chemical form of radionuclides, on the rainfall amount and intensity, and on the type and height of vegetation. The experimental values of the mass interception factor for 131I in particulate form were in general agreement with those derived from the model; when the 131I was in soluble form the values were about 10 times lower than those predicted by the model.

 

TS.3.2. Effective Half-Time of Retention of 131I by Vegetation

After 131I is deposited on vegetation, environmental removal processes combine with radioactive decay to reduce the initial amount on the vegetation surface. The time necessary for one-half of the activity to be removed by environmental processes is referred to as the environmental half-time, Tw. This time value, together with the radioactive half-life, Tr, determines the effective half-time, Te:

Values of Tw may be expected to vary markedly as a function of the growth of vegetation and of meteorological conditions. Given the short radioactive half-life of 131I, however, the effective half-time Te is not particularly sensitive to large variations of the environmental half-time Tw. The median value of Tw was taken to be 10 d, yielding an effective half-time Te for 131I of 4.5 d.

 

TS.3.3. Pasture Intake by Dairy Cows

Fresh pasture is that portion of the cow's diet that is of concern in this study because it is the only dietary component that usually is directly exposed to fallout and can be contaminated by 131I to a significant extent.

It would not be appropriate to use information regarding current dairy practices as a surrogate for dairy practices during the 1950s. The trend towards larger farms, together with the greater daily feed intake required by higher milk producing cows, has led to the increased use of drylot feeding which utilizes little or no fresh pasture.

The only nationwide standardized information source for dairy herd diets is the Dairy Herd Improvement Association (DHIA). The annual summaries of some of the data collected for the herds included in the DHIA program were obtained from the Animal Improvement Program Laboratory, which has maintained since 1953 a national computer database of the DHIA records. By using this database, the pasture intake by dairy cows has been calculated in two steps: (1) the estimation of the total daily dry matter intake of dairy cows, averaged over the years 1953 to 1963, for each of the contiguous states, and (2) the estimation of the fraction of total dry matter intake that was provided by fresh pasture.

 

TS.3.3.1. Total daily dry matter intake of dairy cows

Feeding standards have been established to help farmers in selecting the properly balanced diets for optimum health of their animals and maximum milk production. The daily maximum intake of dry matter (DM), expressed in kg, is estimated using the methodology proposed by the National Research Council, as a function of the cow's body weight (BWT) and of the percentage of cow's body weight to be fed to the cow per day (PBWT):

Values of PBWT are estimated as a function of the cow's body weight and of the daily production of milk normalized to 4% fat content. The values of the fat content of milk (FCM), expressed in kg d-1, vary with the milk yield, MY (kg d-1), and with the fat yield, FAT (kg d-1), according to:

The annual herd averages for cow's body weight, milk yield, and fat yield from each state that were reported to the DHIA during the time period from 1953 to 1963 were used to calculate the average total dry matter intake for the dairy cows in each state.

 

TS.3.3.2. Fraction of total dry matter intake from pasture

The fraction of the total daily dry matter intake by cows which is obtained from pasture vegetation in each state has been estimated on a weekly basis using the expert opinions of individual United States Department of Agriculture (USDA) Extension Specialists across the country and of other knowledgeable persons who were asked to help reconstruct pasture feeding practices during the 1950s. Although subjective, these estimates are the best obtainable information on the seasonal variation of pasture practices at that time. In some states, the environmental conditions, and therefore the pasture practices, varied considerably across the state. To take this into account, two or more pasture regions were assigned to the states of Alabama, Arizona, California, Georgia, Mississippi, North Carolina, South Carolina, Texas, and Utah. A total of 70 pasture regions have been defined for the contiguous U.S.

The daily dry matter intake by cows which was obtained from pasture intake during a given week, PIwk,s (kg d-1), in a given pasture region, pr, (most often consisting of an entire state), was calculated as the product of the rate of total dry matter intake in the pasture region pr, DMpr (kg d-1), and of the fraction of the diet obtained from pasture grass during week wk in the pasture region pr, FPwk,pr :

For each pasture region, a pasture intake estimate is provided in the report for each week of the year.

 

TS.3.4. Intake-to-Milk Transfer Coefficient

The time-integrated concentration of 131I in milk (µCi d L-1) divided by the 131I activity (µCi) consumed by the cow is defined as the intake-to-milk transfer coefficient for 131I and for cows, fm (d L-1). This transfer coefficient has been determined experimentally in a large number of studies, including tracer experiments with stable or radioactive iodine and field studies in which pasture grasses were contaminated by 131I resulting from releases from nuclear facilities or from fallout from nuclear weapons tests. Reported literature values range from 2 x 10-3 to 4 x 10-2 d L-1 but it seems that fallout studies yielded values in the lower part of the range. For the purposes of this report, it is assumed that the median value of fm for 131I and for cows is 4 x 10-3 d L-1.

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