CHEM 316/318
Lab #3
You’ve been hired by a graduate student to run her analyses in a sediment series. You just ran a
calibration curve and the whole series of sediment samples. You need to transform all arsenic
data (raw data) into mass per mass of sample. And you also want to run look at normalize the
data per mass of aluminum (given in percentage) and per unit mass of organic carbon (also given
in percentage).
Calibration data: Lab2(Arsenic)
A) Analytical approach
1) Using Excel, draw your calibration curve and calculate your calibration equation. Is this an
acceptable calibration curve?
2) Transform all your arsenic solution data in mass per mass of sediment (ug/g). You should
know that you injected 1 ml into the instrument.
3) Plot your vertical profile of arsenic in the sediment column (vs. depth – cm)
B) Assessing data variation
4) Verify that Excel calculates the appropriate correlation parameters. Calculate the relationship
between the provided values for the calibration curve by calculating the sum of squares for x
(As signal: SSxx), the sum of squares for y (As standard concentrations: SSyy), and the sum
of cross-products (SSxy).
5) Calculate the correlation coefficient (r).
6) Calculate then the coefficient of determination (r2). Are these parameters the same as those
obtained using Excel?
7) To check if a regression analysis is appropriate, a quick test can be performed, namely the
analysis of residual errors. This analysis involves the graphing of the y residuals (y-y’)
against the x values. If the residuals are evenly distributed and not related to the value of x,
the relationship between y and x is indeed linear. To perform this, first calculate the predicted
y values (y’), and then calculate the difference between the measured and predicted y values
(y-y’). The result is called the residuals. Plot the residuals vs. x values and inspect the
distribution. Is the relationship truly linear?
C) Application: Historical lead (Pb) inputs to urban environments
Introduction:
Lead (Pb) is often considered as one of the oldest humanmade atmospheric contaminants and lead poisoning today
is still regarded as the single most significant preventable
disease associated with an environmental occupational
toxin (Silbergeld, 1995; Mielke, 1999; Mielke t al., 1999;
Ryan et al., 2004; though I would like to argue that
mercury and arsenic are not far behind). Aside from the
long-standing Pb additions to paint products in the late
19th and early 20th Centuries, leaded gasoline has been
recognized as a major source of Pb to the environment
through atmospheric redistribution and concentration
close to roadways and urban regions (Mielke, 1999). The
phaseout of leaded gasoline in the late part of the 20th
Century has led to significant improvements in air quality
and a reduction in a major source of Pb exposure to
humans (see Nriagu, 1990 for details on the “rise and fall”
of leaded gasoline in the US). Because of a concomitant
drop in blood Pb levels of children from urban centers, some investigators initially suggested a
causal relationship and thus stressed the removal of the Pb inputs (leaded gasoline) as the major
remediation effort to curtail Pb toxicity in young children. However, recent research suggests
that although blood Pb levels have continued to decline in the U.S. population, a significant
number of children still have elevated levels (Pirkle, 1998; Mielke, 1999). Scientists argue that in
spite of the successful public health efforts to remove Pb from population-wide sources such as
gasoline and lead-soldered food and drink cans, new efforts must address the difficult problem of
leaded paint, especially in older houses, as well as Pb in dust and soil (Pirkle, 1998; Mielke,
1999; Mielke t al., 1999).
In this lab, we will address the issue of atmospheric Pb levels in a major urban center of North
America: New York City. First, we will reconstruct the recent historical changes in atmospheric
Pb levels using filters collected (and analyzed) over the last 30 years in the city. We will then
compare the trends in ambient air levels with trends in tetra-ethyl Pb consumption in gasoline in
the U.S. Secondly, we will examine a longer time series of Pb deposition in the New York City
metropolitan region during the 20th Century. Based on this latter analysis, we will evaluate the
role of leaded gasoline and other potential sources of Pb to the environment in major urban
systems.
Part 1: Leaded gasoline, a source of urban atmospheric contamination
Please download the Excel spreadsheet “Pb
Atmosph-NYC”. In this spreadsheet you will find
historical data of lead concentrations in
atmospheric Total Suspended Particulates
collected over the last 30 years in the city of NYC.
The data was extracted from a much broader data
repository managed by Columbia University’s
Mailman School of Public Health (World Trade
Center Environmental Contaminant Database). In
part one, please graph the required plots and
answer the following questions.
1) Plot the curve of lead consumption from gasoline combustion in the U.S. from 1970 to
1992.
2) Calculate the average atmospheric Pb levels in Total Suspended Particulates (aerosols)
from 1975 to 1992, and plot that as well on the same graph.
3) Add to this plot all the data from the atmospheric Pb levels in Total Suspended
Particulates (aerosols) from 1975 to 1992.
Questions Part 1: Comment on the temporal trends in gasoline Pb consumptions and Pb levels
in air particulates for the period 1970-1990.
a) Are these trends related? How can you show that graphically and/or mathematically?
b) Are these two trends consistent with information from the literature (your readings,
EPA Air Quality website, etc)?
c) Based on these data, can you make inferences as to what seemed to have been the major
source of Pb to urban environments such as NYC in mid-20th Century (1950-1970)?
Explain your reasoning.
d) How could we reconstruct atmospheric Pb concentrations in urban sites like NYC for
periods prior to continuous air filter sampling?
Part 2: (Only answer this section once you have finished the previous one!). Reconstruction
of historical inputs of Pb to urban environments: The case of New York City.
Well, you’ve guest it. The last question to Part 1 was an introduction to this section of your lab.
Here, we are going to use sediment records collected from the artificial lake in Central Park. The
lake was excavated in the mid-1860s and has had a relatively constant and undisturbed
accumulation of sediments since (Chillrud et al., 1999). Researchers from Columbia University
(Chillrud et al., 1999) have reconstructed the flux of atmospheric Pb to the surface od the lake
over its entire history and have come up with a very interesting discovery. Let’s follow their
footsteps and do their work anew.
In the attached spreadsheet “Central Park Sediments” you’ll find two different ways to represent
Pb measurements in sediments from the lake. Columns A and B provide the depth and age,
respectively, of each core interval. Column D provides “excess” Pb concentrations (meaning the
amount of Pb above the natural background concentrations), in ug/g. Column E, provides the
values for the fluxes of “excess” Pb concentrations to the surface of the lake over time (in
µg/cm2.yr). The last four columns (G-J) give data for municipal solid waste (MSW) incineration
in NYC from 1910 to 1995 (Walsh, 2002), as well as the historical consumption of tetra-ethyl
lead (TEPb) in leaded gasoline in the U.S.
Questions Part 2:
a) Plot the Pb flux to sediments (µg/cm2.yr) vs. date (calendar year) for the period 1900-1990.
b) On the same graph, plot the curve for Pb consumed in gasoline in the U.S. for the period
1910-1990.
c) Based on these data, when is(are) the maximum value(s) of ambient Pb observed in NYC?
Does this correspond to the input values of Pb consumption from gasoline production?
d) Now plot the sedimentary Pb flux (µg/cm2.yr) vs. date (calendar year) of each sediment
interval and add the municipal solid waste (MSW) incineration curve on the same graph.
e) What does this tell you about other potential sources of Pb to the atmosphere in urban
systems (you will need to read the two papers cited above: Chillrud et al., 1999 and Walsh,
2002)? Would you say that leaded gasoline was the principal contributor of Pb to the
atmosphere of NYC in the 20th Century? Explain your reasoning (using quantitative
arguments).
References:
Chillrud, S.N., R.F. Bopp, H.J. Simpson, J.M. Ross, E.L. Shuster, D.A. Chaky, D.C. Walsh, C. Chin
Choy, L-R. Tolley, and A. Yarme 1999. Twentieth Century Atmospheric Metal Fluxes into
Central Park Lake, New York City. Environmental Science and Technology, Vol. 33(5): 658-662.
Mielke, H.W., C.R. Gonzales, M.K. Smith, and P.W. Mielke. (1999). The urban environment and
children’s health: Soils as an integrator of lead, zinc, and cadmium in New Orleans, Louisiana,
USA. Environmental Research (Section A), Vol. 81: 117-129.
Mielke, H.W. (1999b). Lead in the Inner Cities. American Scientist, Vol. 87(1): 62.
Nriagu, J.O. (1990). The rise and fall of leaded gasoline. The Science of the Total Environment, Vol. 92:
13-28.
Pirkle, J.L., R.B. Kaufmann, D.J. Brody, T. Hickman, E.W. Gunter, and D.C. Paschal (1998). Exposure
of the U.S. Population to Lead, 1991-1994, Environmental Health Perspectives Vol. 106(11):
745.
Ryan, J.A., W.R. Berti, S.L. Brown, S.W. Casteel, R.L. Chaney, M. Doolan, P. Grevatt, J. Hallfrisch, M.
Maddaloni, D. Mosby, and K.G. Scheckel. (2004). Reducing children’s risk from lead in soil.
Environmental Science and Technology, Vol. 38(1): 1A–24A.
Silbergeld, E.K. (1995). The international dimension of lead exposure. International Journal of
Occupational and Environmental Health, Vol. 1(4): 336.
Walsh, D.C. (2002). What led to the rise and fall of incineration in New York City? Environmental
Science and Technology, Vol. 36: 316A-322A.
Walsh, D.C., S.N. Chillrud, H.J. Simpson, and R.F. Bopp. (2001). Refuse incinerator particle emissions
and combustion residues for New York City during the 20th Century. Environmental Science and
Technology, Vol. 35(12): 2441-2447.